I'm only in high school and I already have the belly of santa clause. What's the best exercise to lose belly fat?

Sit ups and crunches always worked for me to tone those muscles, but you will also have to start eating right.

It's all about the diet. It's almost impossible to exercise away belly fat. You have to combine exercise (cardio and lifting as well as ab exercises) with a good diet. You don't have to go hungry, but you need to cut out the junk and white carbs. Make a point of getting the most nutritional bang for your buck at each meal. No empty calories, try to get a good portion of protein, healthy fats and veggies carbs at each meal. Cut way back on the refined carbs.

Exercise: Combine cardio (high intensity intervals as well as steady state cardio) with gym work. Concentrate on the big compound lifts like squat, deadlift and bench press. Those burn fat as well as giving you the sort of body you want to show off. Crunches and sit-ups are fine, but you need more to a flat belly.