what’s the fastest way for a 13 year old guy get a flat stomach?

so im 13, around 140-150lb, and 5ft, 7in. and i want a flat stomach. I was really fat as a kid and now its less but i still am kinda. i eat a normal amount of food, i drink plenty of water( dont even drink soda anymore) and i do a 100 sit ups everyday, for a few months, and i don’t see any results.