Do you have any carpeting in the room on the floor. This usually helps quite a bit. Hard surface definitely don't help with the reverberations. Soft materials help absorb the sound such as carpets and heavy curtains.
Also parallel walls create standing waves which don't help either. So you want to break up some of the parallels walls. So think of ways that you can place panels to break this up. Its doesn't have to be too complicated. Even book shelves help. They break up the space and act as a diffusor.
Take a look at the following tutorials to get some great advice of building a studio and acoustic treatment. The advice may go out of your budget, but you'll definitely pick up some handy tips on what you can do.