Briefly summarize your early life in the first sentence. State when and where you were born, and who your parents are. You can use this sentence to say something about how you feel about your upbringing. For example, you could say, "Moishe Incognitovitch was born on April 1, 1979, in Heck, Michigan, to Zippy and Marge Incognitovitch," or you could say "Moishe Incognitovitch was born on April 1, 1979, to the most wonderful parents in the most boring city in the Midwest." If you are doing this for a school assignment, better stick with the first one.
Use the second sentence to give a little bit more information about your early life. You can talk about your parents' professions, your house, your childhood, your school or anything else that strikes your fancy. You could skip all the way to college, or stick with elementary school. The details you choose will help the reader get a better idea about just who you are.
Write about becoming an adult in the third and maybe fourth sentences. Write about where you went to school, where you moved, what jobs you took or what you have accomplished. The sentence, "he worked a series of increasingly bizarre jobs after college before finally stumbling into a cozy position as assistant professor of Romanian Mythology at Miskatonic University" is an example of something both cute and substantive, giving the reader a glimpse of who the author is.
Write about where you are and what you are working on now in the next few sentences. You can name professional projects or life-changing events such as marriage, or you can simply write about hobbies. If you are writing about something trivial, try to be humorous about it. You don't want to let the reader down after discussing the turbulent years of your early life.
Use the last sentence to be cute. It is entirely optional. You can speak to the readers directly, telling them something about how you want to be viewed. For example, the aforementioned Moishe Incognitovitch might end his autobiography with, "He is generally thought to be very nice by people who aren't instantly thrown off by the fact that he spends his life studying vampires."