The squat is one of the very best exercises for the backside. You can do them without weight or with. The key is to squat "ass on the ground low" as my workout partner calls it. You don't REALLY put your ass on the ground, you just squat till you're as low as you can go. If it is hard at first, you can do what is called a "box squat"- use a low seat or bench and squat down to it (as if sitting down) and then back up.
While you squat, you must keep your abs TIGHT and don't let your back roll out.
Oh, and sit up SLOWLY, while squeezing your glutes.