Read as many autobiographies as possible. Attend a creative writing course, or research different styles of autobiography writing. Compile a list, or make a life map of the main events of your life. This should include the obvious life events, like when and where you were born. Make a list of key people, and key places.
Decide whether you want to begin your story before you were born. Some writers begin their autobiographies with their parents or more distant ancestors. Decide whether you want to begin your story at your birth or from early childhood. Some writers find their first or a significant memory a good point at which to begin. Think about the narrative voice you are most comfortable with. Some writers describe the events of childhood from the distance of adulthood, some prefer to describe events as they perceived them at the time.
Decide whether you want to use a strictly chronological order. Consider a more fluid narrative that moves between the past and present. Experiment by writing a short chapter about one event or time in your life. Ask someone you trust, or a writing tutor, to read this and give his opinion. Think about the potential readership for your book, and the length, and whether it fits into any specific category. An example of this would be a sports person writing an autobiography that would be of interest to fans.