If you are very skinny it might just be that you have to eat more food. Otherwise exercise your butt muscles. Many things will do: Running, cycling, walking (especially uphill) etc. The fastest way will probably be to start using suitable exercise maschines in a fitness center (just go to a fitness center, say what you want and they can make a program for you). There are also exercises you can do at home on the floor (like lying on your back on the floor with your legs somewhat bent and lifting up the middle part of the body only by squeezing the butt muscles - very hard in the beginning, but very effective). Unfortunately that kind of floor exercises tend to do harm to your back in the long run or at lest they have to be accompanied by some exercises which strengthens the back.
I hope that's helpful...
