It might be tempting to start your story with "I was born in Dayton, Ohio...," but that is not really where your story begins. It's better to start with an experience. You may wish to start with something like why you were born where you were and how your family's experience led to your birth. If your narrative centers more around a pivotal moment in your life, give the reader a glimpse into that moment. Think about how your favorite movie or novel begins, and look for inspiration from other stories when thinking about how to start your own.
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.