Normalize your eating schedule and amount (eat around the same amount around the same times), which should help it become steady. Eat more carbs/protein.
Working out will also help you.
Most of all, don't abuse your metabolism -- that is, don't eat fast food nonstop. You may not be gaining weight, but you'll still be harming your body if you do. Every once in a while is absolutely fine, of course.
But for now, your best bet is really just to try and eat healthy.
This might help you, too:
http://www.medicinenet.com/script/main/art.asp?articlekey=52231
I am sorry I do not have an answer for you because I'm not sure there is a safe way to slow down your metabolism what you did get a very good answer from somebody else about waiting until your puberty sets in. The timing is always different for each individual and you cannot measure yourself by your peers at school. It would be nice if you could talk to your parents about this or some other adult that you trust so you do not feel so alone.
the metabolism will change by itself once you have gone through puberty, I would not worry that much - think about it, some of those who already have curves at 13 might well grow overweight once they are in their 20s ;)