Lunges - Take a giant step, touch your knee to the ground, take another giant step and touch your knee to the ground.
Squats - Stand with your feet about shoulder width apart and your arms extended in front of you. Bend your knees to a ninety degree angle with your back straight, then rise back up
(repeat a million times)
Those two exercises with build up muscle in your Glutes, giving you a bigger, more shapely buttocks.
Also think about working on your core, (the tighter your stomach is the bigger your buttocks looks.)
Planks - Get into a pushup position, but instead of up on your hands put your elbows on the floor, hold it as long as you can.
(if you get 2 minutes your first time, that would be impressive, so don't give up if you can't do it that long, its very strenuous after about 30 seconds)
Sit-ups I'm sure you know how to do.
a variation of situps is to touch your left elbow to your right knee when you come up, then right elbow to left knee.
You'll feel your sides start to burn within 4 or 5 of those.
(REMEMBER TO KEEP IT EVEN! If you do 3 situps touching your right elbow to left knee make sure you do 3 the other way so you aren't only working on one side of your body.)
Anyways, I think thats all I can say to help.
You could always start skateboarding
