Easiest way to get a flat stomach before summer?

when i was like 11 i lost about 20 pounds. i was close to 100 pounds so i lost weight. im now 13 and i weigh about 100 pounds and i like my weight now....but i dont have a flat stomach. its like i have a "two pack" below my ribs but then the rest is just like blubber...please help me get a flat stomach like my friends before summer!