In order to motivate you to jump out of bed every morning and write tirelessly towards your final chapter, you need a solid idea. This may require a great deal of brainstorming and research, but the more specific you are about what exactly you're writing about, the easier it will be to write. If you're writing a fictional novel, you need to know the beginning, middle and end of your story. If you're writing a self-help book, you'll need to outline the information that you will be covering in each chapter. If you're writing a memoir, you'll need to decide on the theme and how the stories from your life combine to make one solid narrative.
Stories may "write" themselves, but books do not. Completing a book requires that you dedicate to writing on a regular basis, or you will never complete your tome. For some people, this means writing everyday. For others, it means writing on the weekends when they're not required to be at work. In order to finish your book, you must set deadlines that require a certain numbers of pages must be completed by a certain date. Every writer has a different way of working, which is fine, as long as their method turns out results.
The hardest part of writing is said to be rewriting. In order to write a truly great book, one must be willing to re-read and re-work that which is already completed. This process, known as editing, is to ensure that the ideas and themes of the book are clearly communicated in the way the author intended. To help in the editing process, it is useful to ask a fellow writer, scholar or trusted editor to read each draft and give notes in order to help the author create the best possible book.