Write a list of all the information you want to include, not only the basic statistics, such as your previous school records and extracurricular activities, but also your goals, strengths, weaknesses, interests and even a few personal anecdotes you feel will be relevant.
Write an outline for your autobiography. Include a statement for both the introduction and conclusion, as well as main points for three to five paragraphs that make up the body. Each paragraph should have a focus that ties into a theme summarized in your conclusion. For example, if your goal for the autobiography is to be rewarded a scholarship, each point may be based on your financial struggles and how you have overcome them in other areas of your life.
Write a first draft of your autobiography based on the outline. Use first person ("I," "me"). Use professional language with proper grammar and no slang, but also allow your own voice to shine through. Read each paragraph aloud as you work to ensure that it sounds natural, and edit as necessary.
Read the entire autobiography aloud and mark any spelling or grammatical errors you find. Revise as necessary, keeping in mind the flow and overall tone of the paper. You should always have a focus and purpose for each point you make, and you should avoid rambling.
Proofread one final time for spelling and punctuation errors, and then have a few teachers read and critique your autobiography. Revise as needed.