What's your central conflict? What's the biggest obstacle life presented that took years to overcome or come to terms with? Maybe it's an illness you were diagnosed with at an early age, a relationship wrought with turmoil, a series of career setbacks, a goal you worked for decades to achieve, or any other number of things. Look to your favorite books and movies for more examples of conflicts.
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.
Incorporate dialogue into your autobiography to make it more personal, recommends history professor Robert Gerlich at Loyola University in New Orleans. Consider including humor when it's appropriate. For example, if you're writing about how you got lost at Disneyland when you were 10 years old and how the experience helped you learn to face obstacles, problem solve and overcome your fears, you might write, "The attendant at Splash Mountain said, 'Are you OK? Did you get separated from your family?' I quickly replied with as much bravado as I could muster, 'Nah, I'm fine. My eyes are just wet from the ride.' I didn't want her to know I was crying."