How to 'gain' weight when I have a fast metabolism?

Im 13, 5' 1 and im 77 pounds, im too skinny for size 0 at Abercrombie and Hollister. Im the scrawniest girl in the world, and every one calls me anerexic and belemic but im not i eat like a pig and i do NOT make myself puke. i have no fat, no muscle. I think its becuase i have a fast matabolism. But how can i gain weight? i mean i dont want 2 b 'obese' but just a little healthy looking. is there a certain amount of carbs or calories or whatever that i have 2 take in. i eat like 6 meals a day no joke. SHOULNT I B FAT?! help me out!