To put it simply, squats. You see, your buttock muscles are called the Gleuteus Maximus. There are a number of excercises that can enhance the shape of your butt, but only squats, with a relatively heay weight will make the muscle increase in size, thus enlarging your butt. The Gleutes are relatively large muscles, so you need relatively heavy weight for them to grow in size.
To achieve the butt you desire, you must give it your full attention. After all, your big booty will be getting all the attention soon. If you are not born with a big butt, there are still things you can do to get that big booty.