Start by planning out what you wish to say and the manner in which you intend to say it. Outline the important incidents in your life and do research them if you can. Look for old correspondence, check out high school yearbooks and ask friends or family who were present at various events to provide their recollection. Be as honest as you can: unpleasant incidents or incidents where you made mistakes are as important to your autobiography as the more positive stuff. Once you have a solid idea of the facts, you can proceed to the next steps with confidence.
When you have your details ready, plan the organization of your autobiography. Write an outline covering major chapters, incidents and the overall arc in which you choose to present your material. Most autobiographies proceed chronologically--starting when you were a child and progressing through your life--but you can alter that if you feel it makes for a better read. Your outline should briefly sketch out each section of your autobiography, principal incidents and the points you wish to make--no more than a sentence or two for each. You can diverge from your outline while writing if it feels right, but the presence of a basic structure will help keep you on track.
Writing any book is a matter of habit: setting a schedule and then sticking to it. Decide how many days of the week you can devote to writing and how many words you can write each day. Once you have that schedule, maintain it no matter what. You'll always have excuses not to write; don't let them derail you. Sit down and do it. Don't stop until you're done. When you're finished, set the work aside for a few weeks, then go back and revise it. Look for areas that can be improved, rough patches you can polish and dead weight that can be excised. Make two or three revisions, then show the draft to someone you trust: a spouse or an editor who can provide honest constructive criticism. Decide how many of their comments you want to incorporate, then revise your autobiography one last time. When you're satisfied with how it reads, you're ready to find a publisher.