The basic acoustical character of the room is set by its dimensions. It is modified by finishes, furnishings, openings, and such. You also need to consider your floors and ceilings. Treating only the walls may help some, but it's a partial solution at best.
The good news is that it doesn't take that much to make a huge improvement. Most rooms can be done for under $1000 using a kit from someone like Acoustimac. I've used them both in my theater and living room. They use top-qualty nam brand core and cover materials and construction, offer a full palette of colors and textures, and ship quickly.