I am skinny.. about 5"2/5"3.. 15 years old.. and weigh 85 pounds. I know I'm under weight, but I'm trying to lose a little bit of belly fat (even though I have a almost flat stomach). Me and my brother go running sometimes (which helps you get a flat stomach).. And we do crunches and sit ups. Other than that I don't gain much weight from eating (I have a really fast metabolism). Try doing some running exercises, even bicycling can help you. I do horseback riding (seeing how I have my own horse), so I guess I get some exercise from that, because apparently it works some muscles in your body, from staying up-right in the saddle.. Or besides exercise, try to eat healthier