Read published autobiographies of individuals you admire or are curious about, to familiarize yourself with the structure and the advantages and limitations of first-person narrative.
Determine who your target readers will be. If, for example, you're writing your autobiography for people who have never met you, you'll need to incorporate more detail than if only close friends and relatives will read it. If you write, "Emily and I are always fighting with each other," a friend or relative won't be confused, but a stranger will need some explanation of who Emily is.
Decide what your overall theme will be and how it will resonate with your readers. Telling your life story chronologically from birth to age 15, for instance, may be completely riveting to you because you were there but may not interest someone else. If, instead, you focus on the anxiety of transitioning from middle to high school or the frustration of moving to a new town and school where you didn't know anyone, other people may be more interested.
Start with a four-part outline and title the sections Past, Present, Future and Random Musings. You can either compose the elements directly on your computer or jot notes on index cards that you can carry around with you. A binder works well, too, because it allows you to move the content around as you think of new topics to add. The amount of content you're going to allocate to these sections depends on the decision you made in Step 3.
Create subsections under Past such as Birthplace, Parents, Siblings, Grandparents, Ancestors, Childhood, Grade School, Illnesses and Favorite Toys. Write down as much as you can remember that you either personally experienced or learned from others about your family and background as you got older (e.g., you're descended from Vikings).
Ask your parents, grandparents and family friends to help you fill in some of the blanks about why you lived in a particular neighborhood, what your parents did for a living, how you got along with your siblings or other kids, or your first vacation. Take copious notes or use a tape recorder.
Look through family photo albums or scrapbooks for more clues about your background. If you have relatives who like to save everything, you may even be able to find old report cards, refrigerator drawings or blue ribbons for science fair projects. Analyze how these items and events shaped your childhood identity and are still a part of your personality as a teen.
Divide the Present category into subsections: Friends, Sports, High School, Pets, Clothes, Chores, Hobbies, Cars and Crushes. Reflect on the people you interact with every day, the favorite snacks you like to eat, the music you enjoy listening to and the classes that you're taking in school. Even if some entries seem ordinary to you, people from other cultures might find them fascinating if they have never experienced these things themselves.
Create subsections for Future that address topics such as what you plan to do after graduation, where you'd like to travel, what your ideal job might be, whether you envision having children, who your mentors are and what family traditions you plan to carry on as an adult.
Identify anxieties, regrets, daydreams, phobias, habits and spiritual beliefs under Random Musings. Although not all of these may find a place in your finished product, many of them will jog your memory and lead you to recall more events from your life. This is where you can also identify things such as books that have influenced your thinking; qualities that you like or dislike about yourself; favorite possessions; who'd you'd want to portray you in a movie of your life; and what you've observed about love, hatred, grief, fidelity and perseverance.
Start writing and try to keep a consistent schedule so that you don't lose your train of thought. Keep backup copies of your work. Resist editing until you have the first draft completed. Otherwise, you could spend too much time trying to come up with the perfect opening sentence.