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.
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.
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.