Identify your audience. Are you writing this as a record for your children to know and understand your life? As a record for yourself? Are you hoping to have it published and sold in bookstores? How you write and what you record will be different for each kind of audience. Once you have identified your audience, keep them in mind the entire time you write. Imagine yourself talking directly to this "dear reader." What do they know, and what do they already understand?
Brainstorm categories that describe major aspects of your life. Possibilities are: career, love life, life changing moments, personal accomplishments, passions, interests or volunteer activities. Choose three to five of these categories, the ones that mean the most to you and define you the most clearly, to cover in your autobiography.
Think about how you have grown and changed. What made you who you are? This will involve a look over the span of your life from beginning to present. But don't make the mistake of writing a chronological report of your life. It's possible your birth was a major factor in who you are. For instance, you may have been born with a health problem you've had to battle. But don't give an account of your birth, every elementary, junior high and high school experience, every college class, your first uneventful job, ad nauseum. Highlight those experiences and moments that changed and defined you. Not every minute in our lives is worth recording.
Write anecdotes which support your main topics; don't just list details. The names of your first grade teacher or friends are not important or interesting, unless they play a specific part in a story about the time you got caught skipping school and learned an important lesson.
Write your autobiography as a narrative with appropriate dialogue, descriptions, setting and the emotions you felt. Show your readers what happened, don't just tell them about the incident.
Open your autobiography with an interesting anecdote, preview your three to five topics, cover each topic thoroughly with plenty of stories to elaborate, and close with what you have learned or want to share with others.