I love to write. I agree. Each character you create is like a child you want to nuture and care for. That's why it's so hard to kill them off. (I had to before and I knew the character was going to die from the beginning) Fans get mad when characters are killed off but it's harder for the author. ^.^
Maybe you could major in creative writing and minor in art? If you're going to be a mangaka you have to be a great story writer foremost. ^.^
Ummmm your best bet is to do some research. Look for companies that have published younger writers in the past. I know there are sites out there that will publish short stories in a magazine..... Don't be afraid to send in stuff to any publisher. If they read it and love then see you're so young it might help you. they might think, "Wow! Someone so young wrote this???"
J.K. Rowling went to a ton of publishers before Harry Potter was picked up. You just need to send it to everyone. lol It helps if you have an in to the publishing world, but many writers start off like you. Even teens.
So...Don't give up!
~Chero~
Hope that helps.