1. You need to establish your expertise in one area. Once you’ve published 10 short stories about romance or espionage or you produce a dozen nonfiction articles on marriage or saving money, you’re probably ready. You are then an expert on the topic, even if you don’t feel like one.
(My career seems to deviate from my advice, but I started by writing on the topic of marriage and published at least 20 articles. I felt I had nothing more to say so I moved on to spiritual growth. After that I became a ghostwriter and for at least a decade I wrote only books for others. I write in a variety of genres, and I’m an anomaly in publishing. It’s easier to stay in one genre.)
2. For magazine articles, you have to do nothing; for books, you have to do everything. That is, you must promote your books. The more connections you have and the more experience you have in publishing, the easier it is to promote.
3. You must convince book editors that you know your area and that you have connections to promote your books. As you publish articles, you’re learning the craft; as you associate with other writers, you widen your circle of influence. Speaking engagements are excellent. Consider joining Toastmasters or a professional speakers group.
4. Thus, I suggest you find one area that intrigues you and write/sell more articles on the topic. (If you sell only first rights, you can always make those articles chapters of a book with little editing.)
Learn the craft and learn how publishing works before you try to get a book published.