you should start doing cardio exercise 3 to 4 times per week. And then look up things you can do to build your butt muscles. I have had much success! My butt use to be entirely flat, I'm talking not even a bubble. But I read something that said people just don't use their butt muscles for much and you have to engage them. Well I've been exercising and doing butt toning things for a year and butt looks much bigger and rounder. The fat on your tummy will not go away with sit ups or crunches!!! You must do cardio and get your heart beating. And fat on your butt will not make it look nice, it will make it look FLAT and flabby. Work the muscles!