Treating an existing room that is in use.

So I've received the recording bug from a friend of mine that had a Sony MDM-X4 laying around and sold to me for a fairly cheap price. Me and a few guys have been playing in my old room. It's the gateway to the shower, so my parents and I have to walk through it in the morning and at night. My sister sleeps here most weekends when she's in from college. I can't obstruct that pathway. If I could treat it it would make for a better recording and playing room, I think. I've included a rough drawing of the room and it includes measurements. This room is an add-on by my old landlord before we bought it 17 some-odd years ago when I was first born. It might as well be a shed. It drops down about half a foot from the original house. The walls are paneling with normal house insulation. It doesn't sit on a solid foundation. The floor consists of plywood on 2x4s sitting on cement blocks. If I had to I could put sheet rock up, which we may be doing soon anyway. I'm sure the ceiling is just sheet-rock/insulation/plywood/roofing. It has a pitched ceiling, so the wall heights are different, as you can see in the lower right corner of the picture. There's a bit of an echo in the left-side of the room on the picture. What I'm trying to find out is what treatments will improve this room's overall "live" sound and make it sound better for recording? I'm also trying to find out the best way to place a desk that measures 2.5'x5' for my recorder/computer/overall equipment. As an estimate to a budget. I'd say around $500-600 MAX.