Approach the paper with honesty and write in your own voice. Do not try to exaggerate events that have happened to you or make it sound like you are someone else. The paper will come across as stilted and unrealistic if you don't stay true to who you are.
Begin the story at point that makes sense. Do not start in the middle, then go back to the beginning and then to the end. This confuses the reader and makes the overall storyline difficult to follow. Telling the story in chronological order is generally the most effective way to complete this type of assignment.
Pick an overall theme, whether it's events that made you who you are, or how you came to choose a certain school over others. Stick with that theme throughout the story and make sure most of the details or stories you discuss relate to that theme.
Write to your audience. Your autobiography should look different if you're writing for your English teacher than if you were writing for a group of your peers or your mother. Write about subjects your audience would be interested in and avoid those that might shock or offend the audience. For example, you might want to focus on cool, cutting-edge aspects of your life if the assignment is to be read to your peers. If only the instructor is reading it, consider focusing on lessons you have learned from the events that happened in your life.