If this isn't what you want, let me know. I have to add that I can't give a full and complete answer. Half of my how to write book deals with this very question and it took me months to write. Remembering this is only my opinion. It's most important not to start the actual writing of a book too soon.
Most books start with a small idea.
Let it grow in your mind. Make notes of any ideas that come to you and seem to connect with the original idea. Think about the ideas. Usually it's best to start out with the kind of book you most like to read. Think about your idea in connection with these genres.