Use chapter divisions to give the book structure. Chapters are useful because they allow you to move on from discussing certain life periods or events. There's a reason we have the expression that we've "closed a chapter" or "opened a new chapter" in life, and it's even more applicable when discussing an autobiography. Chapter breaks allow you to skip forward 10 years, go back in time, or start describing a new theme without jarring the reader too much.
Develop a central theme, main idea or thesis that clearly explains the purpose behind your autobiography. For example, your theme might be about how you overcame obstacles to become a stronger person, or it might explain why a particular passion -- such as mountain climbing or rescuing animals -- is so important to you. Back your theme with details and background information about your life experiences. For example, you might explain that your dad is in the military, so you've had to learn how to adapt quickly to new schools and unfamiliar environments. Or, you might discuss what it was like growing up in a poor, single-parent home, but how your family was always willing to take in injured or stray pets.