Writing a book?

I've had it in my mind for a little while now about writing a book. However, I don't really know what kind of book I want to write. I know you're supposed to write about what you know or what you're passionate about. For me, I have many passions and know a great many things. So the question is what do I write about? Children's book? Book on being a first time mom? Movies and how they suck nowadays? The highs and lows of being a stay at home military mom? I just don't know, but I want to do this. Suggestions?