Separate your life into a few categories, such as childhood, early adulthood and college years; you need to provide a comprehensive overview of your life in as much length as you've been given.
For each category, think of a few themes that comprised your life at the time. For instance, if in your childhood you had to deal with poverty or sadness, you can use those as themes. If you felt that your young adulthood was filled with happiness or pride, list those themes under the appropriate heading. This exercise will help you to picture your life in a whole new light and realize the themes that have so far encompassed your time on earth.
Look at pictures and other things that will help you remember your past. Pick out a few key moments that truly made you become the person you are today. For example, getting into college can be a very dramatic moment in a person's life; so are the death of a family member and getting a first girlfriend or boyfriend. Remember to keep the themes you came up with in mind when deciding on moments in your life that you'd like to present in further detail for your readers.
Create a timeline of your life, starting with your birth up until the present. Place the special moments you came up with in Step 2 under the specific year they happened. Include in the timeline any other events that are important to your life, and that fall in line with the themes of your life.
Figure out the way you want to present your life's information. You can either write the autobiography in a linear, chronological style--as Charles Dickens did in "David Copperfield"--or you can choose to write in a more fragmented style, starting with one moment and going onto another while filling your readers in on the happenings that came between those moments. Although you must present your full life experience, but that doesn't mean you can't play with different styles of presentation.
Write the autobiography without concern for the typical five-paragraph essay format. Instead, you'll want to be creative and write a narrative that keeps your reader's interest with fascinating anecdotes and musings that answer the question of why you are the way you are. Use the particular style you've chosen in a way that doesn't confuse the reader, and always edit and revise once you've finished writing.