Q: how many calories should a thirteen year old eat

A: That's a little too little info for me to answer. Fill out Mike's Fitness Guide specifying 'Gain Muscle' as the goal (because that's what your body is doing as you grow) to get a rough idea (the diet is at the bottom -- just look at the last line with protein/carb/fat/calories summary). Note that it's only a rough guideline, not a prescription by any means. The only real rules you should follow as far as your diet at your age is to eat healthy stuff with lots of veggies and fruits and not lose weight quickly (or at all actually, unless you're significantly overweight).