I personally do not know how to enter the publishing world but you might try to contact the author Nancy Yi Fan. Her first book "Swordbird" was published when she was twelve (or around that age) too. She might have some suggestions.

I have deja vu. I'm sure I answered this question of yours before, but I'm not sure where my answer went.

There are several publishing companies you can submit manuscripts to, but keep in mind that publishing companies are much harder on your work than your family and friends are. You'll be rejected by the majority of them. Don't take it to heart-- so was JK Rowling.

But I want to stress that you cannot publish fan fiction (like your Twilight story) because it is copyrighted material. You can't make money with someone else's universe and characters. Moreover, you shouldn't.

You have bright ideas in your head. Use those. Refine them. Work on them. Don't jump the gun. Reread them. Edit them. Cradle them like children, and they'll grow into great stories. The more you work on a story, the more likely a publisher will agree to publish it.

I thought you could pulish at 13...? I thought I saw a news story about a little 5 year old who got his book published ^_^