How to get a Flat Stomach in 3 months??!?!?!?

I am 14 year old guy, overweight I guess, 5'6" 155 pounds. I need to get a flat stomach and flat chest by June 15 or so give or take a few days for the beach and pool and stuff. I can do exercise and whatever it takes but I am not the healthiest eater. For breakfast I eat a nutrition bar or yogurt with a big gl of orange juice. For lunch I eat whatevers for lunch that day and sometimes a bag of chips or pack of cookies occasionally or fruit snacks. And for dinner I eat Lean Cuisine maybe a day or two a week or what my grandma fixes for dinner which is usually not too bad for me. I also drink water or Arnold Palmer Zero Tea. Is this too unhealthy? I am not looking to bulk up. I am completely focused on getting a flat chest and stomach. I am going to train for sports and lift weights and do speed training in the summer. Please tell me what I need to do!! Thanks.