tips on writing a manga story?

ok, this may sound stupid, but im writing a book, but at the same time I am learning to draw manga... so after I finish this book I want to turn it into a manga and get it published by either the viz or the company that did Rosario+Vampire (if different haha) I have characters, but I might need more, I feel a little short of characters and story ideas... I have a basic idea. But it would be nice for a few side quest ideas haha