Before you begin writing your book, you must research your idea and see if it is in-demand with readers. Who is going to read it? To whom are you trying to appeal with your words? You must have a general idea of who your intended audience will be. Check out other books. Is there a book already published that resembles your book? What will make your book unique from theirs? If there are similar books already out there, what is going to make your book different and make people want to buy it?
When you are ready to begin your book, decide on a schedule that is best for you, one that you can stick to. Your schedule should begin before your research and carry through to the completion of the book. Make a detailed outline with the main plot, events leading to that plot, and explicit detail about the characters. An outline is also a good reference point to double-check your timelines and details.