Consider your audience. Any book, particularly an autobiography, is written for an audience. Your readers may be colleagues in your specialized field, they may be close family members, or they may be a more general public. Each audience will have different interests and respond to different portions of your life story. A professional audience may not care about your wedding, while a family readership may care less about your career details. Think carefully about who is likely to be reading this autobiography before beginning.
Collect your best stories. No autobiography can comprehensively tell a life story. Every one of us experiences far too much to fit into any single book. Rather than try to compress everything in chronological fashion, consider instead some of the best, most memorable stories from your life. A good autobiography should not be a dry observation of a procession of events, but a look at what kind of person you are and how you experienced life. Without worrying about how it all fits together, jot down your favorite, most interesting memories.
Organize your stories. Put each of your favorite stories on a note card. Then organize the note cards chronologically. These should provide the rough structure of your autobiography, but they may also reveal gaps. Are you missing some material from a significant portion of your life? Does this gap jog loose any interesting stories you may have forgotten. Consider talking to family members or old friends about their recollections from these periods. But if there's nothing interesting, feel free to move quickly past that period; some of the best biographies do. For example, Henry Adams' autobiography skips 20 years in the middle of his life.
Outline your chapters. With the note cards organized and grouped by period of your life, plan the chapter by chapter outline of the book. Perhaps you only need one chapter on your childhood, but perhaps you have sufficient material for two or three. Let the strength and quantity of your material dictate the chapters. Typically if you have fewer than ten chapters or more than 25 you may need to make some adjustments. With this chapter structure set, you can begin writing your autobiography.