How can I make my pancake into cakes?

How can I add a little more meat to my backside lol? I'm currently 15 1/2, 118 pounds and a D cup, and I am just lacking in the area. I've noticed in my family that the women have a nice behind but only if they are about 145+ and beyond, like my mother. I would only go up to 125 pounds to attain my goal. What foods/ workouts do you reccomend? Thank you(: