What can I do to gain a butt?

Exercise. Everyone's body is different. Fat is distributed to different places in & muscle mass varies between different people. However, toning your muscles can help. Doing exercise like squats & lunges can help develop your glutes (butt muscles).
Stairs. Using a stair stepper or similar type of exercise will help to shape the gluteus muscle group. Look for exercise regimens that specifically claim to shape your backside.