I had that problem too when I was 12-14. I think a LOT of boys do actually. I ended up being 6′4” and I was very slim by the age of 15. You may not end up that way but here is some advice regardless.

Eat as much as you want. You are still growing and you will likely get taller which will stretch out of round tummy you got there. You could embrace it too. My little cousin is 12, has round belly and wears his belly proudly. He pats his belly at the pool after he eats some hotdogs and says "Gimme 10 minutes and I’ll be at it again" and people laugh. They love having him around because he makes the fat people more comfortable. In a few years these days will seem so petty! Don’t worry so much about it man.

As far as losing weight fast: Eat ONLY fruits and vegetables for a whole week. CARROTS and CELERY are fantastic. If you don’t hjave the willpower to do that then eat FISH instead of beef. Or chicken if you can’t find any fish. But definetly eat carrots and celery. Broccoli is good too if u steam it. Good luck and be proud!