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.
Sometimes people come into your life and you know right away that they were meant to be there, to serve some sort of purpose, teach you a lesson, or to help you figure out who you are or who you want to become. You never know who these people may be (possibly your roommate, neighbor, coworker, longlost friend, lover, or even a complete stranger) but when you lock eyes with them, you know at that very moment that they will affect your life in some profound way.
Create an outline or a graphic organizer of the major topics and ideas you want to cover. For example, you might make a time-order chart -- a chart with event blocks to organize events chronologically. Or, you might draw a cluster web -- a diagram with connected circles -- to divide your autobiography into sections, such as one for relevant background information, one for life-changing events, one for special memories and one for goals and dreams. Focus on reasons why specific events, situations and circumstances were important to you and how they made you feel.