Nick's advice is good, except that you need to be able to identify the problems, and if you are just starting out that may be a problem. Or maybe not? I'm not sure about your experience.
There are four "tools" at your disposal:
1) Room geometry - this is where I start, even for an existing space. And if you'll excuse my arrogance, I believe it is where every project should start!
- Small rooms are not (despite repeated reports to the contrary) statistically reverberant. Treating them with solutions that are intended to treat reverberant spaces does not work.
- What does work? Start with symmetry from left to right. Next experiment with loudspeaker (and ear) placement. I think you will be pleasantly surprised with what you can accomplish with these tricks.
2a) Absorption - turning unwanted sound energy into heat. Yes, that's what it is. There are numerous products that claim to provide aborption - foam, rock wool, even tuned membranes.
- I think panels made from rock wool look nice, but some folks like foam. Rock wool panels are typically a little more efficient, but it depends a lot on how they are made.
- One important caveat - too much absorption is just as bad as too little.
- Another important caveat - absorption needs to compliment the room, for example, if the room is bass heavy then you need more low frequency abso
- Last caveat - tuned absorption is a wonderful tool, but it is really difficult to do well! Start with broadband absorption!!
2b) Absortion (again) - managing reflections. In my limited experience it is very difficult to adequately control reflections with absorption. It can be done, but there are better ways. AND, it almost always results it a room that is too dead.
3) Diffusion - the idea is to break up the reflections and scatter them around the room so they are not distinct. It is remarkably effective in a space that is designed to take advantage of diffusion from the start. You really need about 12 feet from the ears to the rear wall to make diffusion effective.
4) Reflection - not sure why so many folks skip over this, it can be a really effective tool, especially in smaller spaces. The idea is simply to lengthen the path that sound travels to manage the arrival times of different reflections.
None of this helps with isolation or "sound proofing". That's a separate topic, and one you are unlikely to be able to control without a lot of money.
None of this addresses ergonomics, or lighting, or power, or half a dozen other things you need to think about. Again my experience only, but dealing with these topics first constrains some of your acoustical treatment choices. That's not always bad!
No one can offer you specific advice without detailed drawings, or being there. Years (decades?) ago I worked remotely on a couple of rooms. I won't do that anymore, it worked, but it took a lot of time, and there was more than a little frustration on both sides.
And while I can't (won't?) offer specific advice, and I shy away from product recommendations I will suggest one solution. ASC makes these bass traps that double as monitor stands. They are brilliant. They are expensive, but I think more than worth the cost in most rooms. They are definitely worth at least a look.
You may have noticed that I haven't talked about measurements, or room correction. Measurements are wonderful, if you know what they mean. If you don't then you will probably be better off without.
Room correction, on the other hand, borders on snake oil! Not because it doesn't work, but because it does not work the way the vendors suggest. You can not fix physical problems with filters, no matter how narrow you make them, or how many you have. It is the wrong cure, and if you start down that path you will be disappointed.
What it can do is mask specific problems in a specific spot in the room (usually your listening spot). Move a little as a few inches away and the problems could get worse. It is a reasonable alternative to mixing in headphones I suppose, I don't care for it. It has been a while since I've tried any of the products, but last time around they all left sigificant artifacts that - to me - were worse than the problems they were solving.
There are studio designers out there who can give you a great design, even remotely, but they are not cheap, and I'm not sure I'd spend that kind of money on a temporary space. If you are building a more permenant space then by all means consult with one.
Be careful! While it is important to treat the space, and spending a reasonable sum on that solution is worthwhile you want to be careful not to spend unnecessarily.