Organize what you've written according to cause and effect and action and reaction. Ordering events according to cause and effect is a natural process. Your car broke down, so you called a tow truck. That's cause and effect. You received a promotion, so you bought a house. Action/reaction happens paragraph by paragraph. He said "No," so you said "Why?" That's action/reaction. She set a wet glass on the table, so you placed a coaster under it, and remembered how your mother used to crochet pretty little coasters and starch them. That's action/reaction.
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.
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.