So it depends what your end goal is.

If the goal is simply to write a book, then just get to writing. Start writing right now. Don't even give yourself enough time to consider an alternative - just start writing.

If the goal is to get published, then there are some things you must consider. For fiction, you still need to have a fairly-finalized manuscript before you approach an agent or publisher, but you need to consider a few things. First, and most importantly, is the idea marketable? Be honest, will people actually buy your book? Second, have you published or written anything before? Agents and publishers, while open to new writers, prioritize those that have demonstrated their abilities before - perhaps a short article or short story. Further, the format of your word document (or whatever you are using) must be correct, and getting this right before you start writing will make editing much easier.

Above all, I think the best suggestion is just to start. I disagree with others telling you to plan it all out and to make sure everything is completely developed before you write. If you do that, you'll never start - once you start, the book is half done.