How much should I eat a day?!

I'm a 13 year old boy and I've been wondering how much I should eat a day. Most of the time, I eat less than 1500 calories a day, and I exercise enough to take off at least 500 calories each day from my total but I don't feel hungry at all. I have a 6 pack but I want it to be more defined and I've heard if I eat less than I should, my body actually retains the fat and burns muscle. Should I continue eating like this? I've heard that eating 5-6 small meals a day is better. If I do need to eat more, about how many more calories should I? Any tips on exercise to define my abs?