The squat is one of the best exercises you can do, not only will it help tone your bum but it's a very functional movement that, if done right, will help you in everyday life too. This video illustrates squats with a barbell, but you can do them without any weights too.
If you're just starting out, use a ball against a wall for a ball squat to support you and get you into the right position. Place the ball in the small of your back with your legs hip-distance apart and slowly lower down.
Lunges are another great lower-body exercise that work not only your bum, but also your quadriceps muscles too. This video illustrates ball lunges using a ball against a wall but you can also do them free-standing and when you're comfortable with that you could even add in some weights.