If you want to be a writer, you have to write, and not just think perpetually about writing after you get your so-called best idea. Start small. First, write a 10-word post. Write a few more posts. Next, write a short article, then a long-form article, and then a few more articles that can get you in the groove.
Eventually, you'll build the momentum to outline and structure a short book. Next, you'll gain the confidence and the muscle to write a full-fledged book, and even a series of books. Well, the end goal of writing is not necessarily always a book, however all forms of writing are a way to think, structure and express our ideas.
BTW, if you're not comfortable posting online, don't. The point is you have to write, no matter what.
Starting small isn't thinking small.
Start writing and thinking; instead of thinking about writing.
Writing is a muscle.
Write!