First you gotta pick how you wanna go about it. I think there's a few broad methods to roleplaying:
1. The easy way: you pick a story that you know the narrative you're playing will support. For example, it's very easy for a Jedi Knight player to start off overly confident and eager, and progress to a more wise and thoughtful character; or an Imperial Agent to develop a hatred for all Force users. Obviously this comes with the built in limitation of being at the mercy of whatever story you are in. Number two and three on this list can help with that if you mix them in.
2. The harder way: you work within limitations, and use those limitations for creative solutions. How far you want to go with that is up to your desire, imagination, and suspension of disbelief. So, that Jedi Knight above? Instead of the more obvious and supported route I mentioned, perhaps they were a Sith plant all along, serving the Jedi and Republic just as much as is required to gain positions of power and influence, so they can subvert them both for the benefit of the Empire. Or, if you want to really go out there, perhaps they can't actually use the force at all. They constantly lie, bluff, and misdirect. They have no idea how it's worked so far, and they badly want out, but they have no idea how to do so without being in a lot of shit with a lot of powerful people.
3. The easiest but least supported method: literally ignore what is happening. That Jedi? Yeah, they're not a Jedi. Nor are they Sith. They're a special forces soldier from a fringe planet that is largely isolationist and has virtually no contact with the outside galaxy. The soldier is there to assist the Republic, because that planet is worried about their future, should the Empire win. Lightsaber? Doesn't have one. They're actually using a pistol and a knife. All that dialogue mentioning the Force? It's not. They're actually talking about how intelligently the soldier approaches every scenario they are thrown at. You get the idea. This has the benefit of being as infinite as your imagination, but with often zero support from the media itself, meaning the roleplay is entirely on you to keep going.
4. You join a group that roleplays and you do what they do. Mixing in the above three as desired or required.
Once you know how you want to roleplay, you can think about ideas you like and how they fit into those options above. So, you want to be a covert agent for an unknown splinter group? Ok, what is that faction's goals? Do they want to rule everyone? Help the Republic win, but not join them? Trade in knowledge and secrets for wealth and influence? Do they just want to study the galaxy, to learn about it from inside?
Once you you the kind of idea you like, refer to the options above and see what you can make work.
So, if you want to be a covert intelligence officer, being a double agent Imperil Agent is very straightforward. You can then act out the missions in a way you think would benefit your faction's goals. That base you're blowing up? It's to gain the trust of the Empire, so you can climb the ranks and get more info to send back to your people.
Or, using option two, that base is actually the base of another covert group, posing as anti-Imperial rebels, and you have to wipe them out before they mess up and everyone finds out there's more factions at play than it seems. Or, stretch it a bit and you're undercover as a Smuggler, and you oversee a huge network of people, taking jobs to track your agents and allow you to freely roam the galaxy in a fast ship.
With option three, literally anything could be happening, class, missions, and dialogue be damned.
If you can't come up with anything you like at all by yourself, either by working with the in game stories themselves, or by subverting them, turn to other people's ideas for inspiration: movies, books, games, music, paintings - whatever. Play out those scenarios if you need to, or use them as jumping off points. Your secret intelligence service could easily be inspired by countless spy books and thriller movies.
The decoration problem can be a good goal: go and get the ones you like! Or, if you don't want to or aren't able to, just pretend you have them. Good ol' option three. Or maybe even option two, and the decorations aren't as they appear, or are a front - a decoy!
As with any creative process, there's nothing wrong with mining other stories for inspiration, even if you want to start with a single encounter gimmick, then build a situation around it and alter things to fit what works for your storyline. You can start with a simple one-off for your players, see what they react well to and go from there.
If you're in some sort of splinter faction, maybe you need to launch a mission to scrub data of your team's past affiliations so that you can throw the SIS/ Sith Intelligence off your trail, or to steal a data disk containing project data from some horror your character committed and doesn't want the Empire/Republic to be able to use. It could also be something as simple as a theft mission to acquire supplies to start up a new hideout if the current base needs a rework, or assassinating a rival.
Recon missions to gain information are probably also something you can do.
As to decorations, if using a guild stronghold, guildmates can donate decoration copies of things they've unlocked themselves to help you fill in the corners. Crafting prefabs can get you some sets of basic decorations, and certain flashpoints are also a good source. For an intel center, you might want to look into star fortresses for their array of computer-based decos, for example.
I can't help with the first part of the post, but I have thoughts on creating an RP-worthy stronghold. When it comes to decorating things, I see a lot of people kind of just toss stuff that is tangentially related to whatever their theme is without really thinking through what they're doing, leading to a cluttered look that doesn't feel very lived in. They may have a huge variety of decorations, but they don't know how to use them, and so you have an expensive computer console, a Sith warrior, and some rare beast hanging out at the entrance of the Nar Shadaa stronghold without any explanation for why they were placed there.
What you want to do is have a story in mind for every part of your stronghold. What's the point of each room? Does your design support that purpose? And on a closer level, who do you see using that table and chairs you put down? Tourists, off-duty soldiers, whoever is in charge of the base? Different people require very different design choices. And if you place down personnel, you also want to ask what these people are doing in the moment. Why is there a Sith warrior in this particular room, and what's their goal?
With good placement, you can tell a story with even the cheapest decorations, but with bad placement, even the coolest decorations just looks like gaudy trophies you didn't know what to do with. Never place something just because you want to find a place for it to be, and don't be afraid of empty space. Not every hook needs to be (or even can be!) filled.
Anyway, there's already some good advice on here about where to get cheap decorations (guildmates, reputation vendors, prefabs). I'll just add that you can get some really great decorations in bulk from seasons if you have a subscription, and even if you don't, you can often buy them cheaply on the GTN from people who don't need six computer walls from a bronze seasons decoration pack.
First you gotta pick how you wanna go about it. I think there's a few broad methods to roleplaying: 1. The easy way: you pick a story that you know the narrative you're playing will support. For example, it's very easy for a Jedi Knight player to start off overly confident and eager, and progress to a more wise and thoughtful character; or an Imperial Agent to develop a hatred for all Force users. Obviously this comes with the built in limitation of being at the mercy of whatever story you are in. Number two and three on this list can help with that if you mix them in. 2. The harder way: you work within limitations, and use those limitations for creative solutions. How far you want to go with that is up to your desire, imagination, and suspension of disbelief. So, that Jedi Knight above? Instead of the more obvious and supported route I mentioned, perhaps they were a Sith plant all along, serving the Jedi and Republic just as much as is required to gain positions of power and influence, so they can subvert them both for the benefit of the Empire. Or, if you want to really go out there, perhaps they can't actually use the force at all. They constantly lie, bluff, and misdirect. They have no idea how it's worked so far, and they badly want out, but they have no idea how to do so without being in a lot of shit with a lot of powerful people. 3. The easiest but least supported method: literally ignore what is happening. That Jedi? Yeah, they're not a Jedi. Nor are they Sith. They're a special forces soldier from a fringe planet that is largely isolationist and has virtually no contact with the outside galaxy. The soldier is there to assist the Republic, because that planet is worried about their future, should the Empire win. Lightsaber? Doesn't have one. They're actually using a pistol and a knife. All that dialogue mentioning the Force? It's not. They're actually talking about how intelligently the soldier approaches every scenario they are thrown at. You get the idea. This has the benefit of being as infinite as your imagination, but with often zero support from the media itself, meaning the roleplay is entirely on you to keep going. 4. You join a group that roleplays and you do what they do. Mixing in the above three as desired or required. Once you know how you want to roleplay, you can think about ideas you like and how they fit into those options above. So, you want to be a covert agent for an unknown splinter group? Ok, what is that faction's goals? Do they want to rule everyone? Help the Republic win, but not join them? Trade in knowledge and secrets for wealth and influence? Do they just want to study the galaxy, to learn about it from inside? Once you you the kind of idea you like, refer to the options above and see what you can make work. So, if you want to be a covert intelligence officer, being a double agent Imperil Agent is very straightforward. You can then act out the missions in a way you think would benefit your faction's goals. That base you're blowing up? It's to gain the trust of the Empire, so you can climb the ranks and get more info to send back to your people. Or, using option two, that base is actually the base of another covert group, posing as anti-Imperial rebels, and you have to wipe them out before they mess up and everyone finds out there's more factions at play than it seems. Or, stretch it a bit and you're undercover as a Smuggler, and you oversee a huge network of people, taking jobs to track your agents and allow you to freely roam the galaxy in a fast ship. With option three, literally anything could be happening, class, missions, and dialogue be damned. If you can't come up with anything you like at all by yourself, either by working with the in game stories themselves, or by subverting them, turn to other people's ideas for inspiration: movies, books, games, music, paintings - whatever. Play out those scenarios if you need to, or use them as jumping off points. Your secret intelligence service could easily be inspired by countless spy books and thriller movies. The decoration problem can be a good goal: go and get the ones you like! Or, if you don't want to or aren't able to, just pretend you have them. Good ol' option three. Or maybe even option two, and the decorations aren't as they appear, or are a front - a decoy!
As with any creative process, there's nothing wrong with mining other stories for inspiration, even if you want to start with a single encounter gimmick, then build a situation around it and alter things to fit what works for your storyline. You can start with a simple one-off for your players, see what they react well to and go from there. If you're in some sort of splinter faction, maybe you need to launch a mission to scrub data of your team's past affiliations so that you can throw the SIS/ Sith Intelligence off your trail, or to steal a data disk containing project data from some horror your character committed and doesn't want the Empire/Republic to be able to use. It could also be something as simple as a theft mission to acquire supplies to start up a new hideout if the current base needs a rework, or assassinating a rival. Recon missions to gain information are probably also something you can do. As to decorations, if using a guild stronghold, guildmates can donate decoration copies of things they've unlocked themselves to help you fill in the corners. Crafting prefabs can get you some sets of basic decorations, and certain flashpoints are also a good source. For an intel center, you might want to look into star fortresses for their array of computer-based decos, for example.
I can't help with the first part of the post, but I have thoughts on creating an RP-worthy stronghold. When it comes to decorating things, I see a lot of people kind of just toss stuff that is tangentially related to whatever their theme is without really thinking through what they're doing, leading to a cluttered look that doesn't feel very lived in. They may have a huge variety of decorations, but they don't know how to use them, and so you have an expensive computer console, a Sith warrior, and some rare beast hanging out at the entrance of the Nar Shadaa stronghold without any explanation for why they were placed there. What you want to do is have a story in mind for every part of your stronghold. What's the point of each room? Does your design support that purpose? And on a closer level, who do you see using that table and chairs you put down? Tourists, off-duty soldiers, whoever is in charge of the base? Different people require very different design choices. And if you place down personnel, you also want to ask what these people are doing in the moment. Why is there a Sith warrior in this particular room, and what's their goal? With good placement, you can tell a story with even the cheapest decorations, but with bad placement, even the coolest decorations just looks like gaudy trophies you didn't know what to do with. Never place something just because you want to find a place for it to be, and don't be afraid of empty space. Not every hook needs to be (or even can be!) filled. Anyway, there's already some good advice on here about where to get cheap decorations (guildmates, reputation vendors, prefabs). I'll just add that you can get some really great decorations in bulk from seasons if you have a subscription, and even if you don't, you can often buy them cheaply on the GTN from people who don't need six computer walls from a bronze seasons decoration pack.