rita

A stairstepper is a good cardio workout. If you do it with enough resistance you can also help your butt get toned (and some say it may even help to get it bigger). If you step with the heel off the step, your calf muscles will get a nice workout and may also grow and get more shapely and defined.
Using the ankle weights, put both (I am not sure how heavy yours are) on one leg and do kickbacks. This will work your glutes and if weight is heavy enough, it wll make the muscle grow. Also, you can do lunges (put the ankle weights on). This will help to tone and build nice thighs and help your glutes as well. You can also buy resistance bands (like huge rubber bands) in a sport store and use these in place of weights to get some musclebuilding going on. They are light, portable and not expensive. Some sample exercises usually come with the bands [removed]
Make sure you eat lots of good, nutritious food. Don't just fill up with calorie-rich junk food. It's really not good for you. You do need to eat a lot in order to build muscle mass whn you are doing weights but it should be healthy food.
You can do exercises for your glutes using the big inflated exercise balls.
Lie down on the floor, your legs straight, and put your feet up on the ball. Then try to make the ball come closer to your butt by pressing on it with your heels and rolling it towards you. Do 8-10 reps, 3 sets, 3 times a week. Your glutes will thank you (after they stop hurting you will be thankful too).
Good luck and let us know how you progress (and you will).