I'd start with what kind of book you'd want to write, and read as many of them as you can. If you can't read them, you probably can't write them. If you don't have a specific story in mind, perhaps you can practice (and test yourself) by taking a story in your native tongue and try to translate it into Japanese.
Consider why you want to write in Japanese; is it because you just want to practice the language, or is there a deeper reason? For instance, when I read Haruki Murakami's books, I'm inspired by the magical realism he uses, and it makes me want to write similar stories, but without a strong preference for language; when I read Kotaro Isaka's books, I'm more inspired by his prose--the way he expresses himself--and it makes me want to emulate that style specifically in Japanese. So if you're in the latter camp, what about Japanese allows you to tell the story/express yourself better?