Unless you intend to write a full biography or autobiography, your book should not cover every single detail of your life. Instead, find a single theme or angle in your life on which to focus. For example, your book may be focused on your battle with an addiction, how you rose through the ranks at your job or your extensive traveling experience. In this case, every scene and event you choose to include in your book should relate to that theme.
Every book begins with an outline, although you can go about writing one in several different ways. The outline can be as detailed or vague as you'd like, and you might find later that you stray from it, which is fine. But in the beginning, it is helpful to have something structured to help get you started. Try taking a set of notecards and, on each one, writing down a few sentences describing a memory or event you want to include in the book. When you finish, shuffle the cards up and lay them out flat. Try arranging them in any order. While a biography is typically told chronologically, a memoir and other types of true life books don't have to be, and you may be able to find a different order that is more effective for your book.
Even if a book is your own personal true story, you still need to conduct research or you run the risk of inaccuracies. Much of what you write will be based on your memories, which are not only subjective, but may also be hazy. For example, before writing that you loved a certain cartoon when you were six, do the research to confirm that the show was on the air that year. Doing the necessary research in advance helps you avoid the embarrassment of publishing a memoir or book with errors.