Was just wondering what would be the best kinda of wood to use for the sides and back .I don't want to get to expensive .

Any type of wood e that is treated will work.

Any type of thin plywood will do. I have used luan flooring underlay, which is a hardwood 1/4 inch plyboard. I have also used a cheap grade of paneling, I just put the finished side inward. As long as you put a good coat of paint on them, it will weather really good.

I used OSB sheets and I wouldn't recommend them. They're fine for a few years but they start to peel and separate even when painted. I'm replacing them with regular 3/8" ply for the walls and 1/4" for the floor. You definately want pressure treated for the frame. Check for warped and knotted rejects at Lowe's/Depot. You can get them cheaper.