A flat butt can be caused by a number of lifestyle factors, including sedentary jobs or activities that require you to sit for extended periods. As you age, your butt may flatten and lose shape due to lower amounts of fat in the buttocks.
Squats overall are an amazing workout for the legs and glutes. Every fitness professional around tends to prescribe squats for leg strength as well as for calorie burn, since working your larger muscle groups burns more calories.
Learning the basic squat is your first goal on this front, but once you have the basics, it is important to mix up your workouts and add dimension to the moves.