Good websites where I can write books/stories for free?

Now I don't want to publish anything, I'm only 13. I just need a website that's easy to navigate and control. I don't want a little kiddie site though, I'm very serious about my writing, I want to become an author when I grow up, lol. I already have an account on Quizilla, and pretty much every well known website like that. I just need a good website that isn't too popular, but still where people can rate and/or comment?