A personal biography should typically contain information on your family background, childhood and adult life, and it should have an overall theme. Writing an autobiography requires recognizing which aspects of your life make it unique. Most people have far more interesting life stories than they realize; it's just a matter of determining what is unique about your life.
Most biography writing typically starts with the subject's birth. However, rather than a simple statement of when and where you were born, it's a good idea to think back further, examining the circumstances of your birth. Where did your parents come from? How did they come to settle in the region where you were born? What is distinct about that area?
An account of childhood typically comes next in a personal biography. Although many people find little of interest in their childhoods, there are often specific events that inform the course of a life. Similarly, the best personal biographies trace a theme that runs throughout the life. Closely examining your family history, childhood and adulthood may yield a better perception of how they all fit together. Perhaps there are connections between your family's story and your own. Maybe you have been pursuing a goal since childhood that you only realize in the course of writing your biography. Looking at your life as an outsider is an effective way to perceive new aspects of your story.