What type of exercises should I do?

Do squats. Focus on squeezing the glutes. Most people say to squat until your thighs are parallel to the floor. This is for people who use heavy weight. Scientific studies show that the further you go past that point, the more your gluteus muscles are activated. Try Squatting with just the bar and go all the way down until you are completely squatting. If the bar is too much, do it without the bar. And vice versa. Add weight as needed. Just don't go too heavy, as going past the parallel point will put a lot of stress on the knees. Of course follow typical squat techniques. Head up, chin out, back straight.

UMM.I THINK U SHOULD DO YOGA/PILATES/KICK BOXING AND ALL THAT.ITS FUN AND AT THE SAME TIME IT KEEPS U IN SHAPE.ESPCIALLY TO TIGHTEN UR BUTT, U CAN USE THOSE BIG BALLS AND EXCERISE WITH IT.U CAN ASLO RENT OR BUY MOVIES THAT'LL SHOW U STEP BY STEP EXCERISES FOR DIFFERENT THINGS..ITS REAL COOL.IM DOIN IT IN GYM CLASS AT MY SKOL.HOPE THAT HELPED.

pilates,yoga do not use weight that is an old lie after a while without practice with the weights it will be worst.

no dont buy a machine its ti expensive do leg lifts and situps.