I want to be a writer when I grow up, but I don't know where to go from here....?

I'm 13, and I want to be an adult or young adult science fiction/fantasy writer/novelist when I grow up. I've been working on improving my grammer, spelling, writing skills, etc, up to the point where I am now, and now I don't know where to go from here. I get all these great ideas, but when I go to write them down, the words just don't come. Does this happen to all aspiring writers/writers? Advice would really be nice