Yo I actually had to do this for my professor last semester. I followed this format:
What should I write: An autobiography is, by definition, the story of one's life – that is basically what you should write (remember your life didn't start in high school). Unlike the limited space in which you must write a personal statement when you apply to professional schools, this essay can be as long as five pages, but must bat least two full pages. Note that it is for the use of the Health Professions Office and Committee and is not forwarded elsewhere. The points to cover in your essay are as outlined below:
Your Chosen Field: When, how, under what influences did you decide to pursue a career in medicine or other field? Did some special event or circumstance influence you? What have you done to become better informed about your intended field? Have you done volunteer health care work, had a paid job in health care, or shadowed a professional? Mention family influences (i.e., my Mom's a physician) or even a sick relative or friend for whom you have provided care.
Research Experience: It is not necessary to have done research as an undergraduate, but a great many students have participated to a greater or lesser extent. If you have carried out research or will do so in the coming year, provide a brief description and state what you gained or expect to gain from it. If you have done an honors project, either departmental or college, note it. If you will see your name on a publication, state in what journal. Be sure that you fully understand what you have done and the reason for doing it. Don't be embarrassed in the future by a question about your research which you cannot answer, or discuss from a position of knowledge.
Community Activities: Your community includes college as well as your home town. Talk about your clubs and activities. If you have held leadership positions or taken a leadership role in some activity, describe it, but don't try to glorify yourself. Write about your health care experience – it is important to know that you have been in a professional environment and can imagine yourself in that setting in the future. Other community activities also "count", service in your church, school, local service clubs, and so on.
Recreation and Entertainment: How do you spend your free time? Do you have a life? It is necessary that you present yourself as a well rounded person with interests in something other than high grades and good MCAT scores. Do you read? What was the last book you read that was not required for a course? Are you physically active? Don't try to be all things, but be more than simply a strong student. Interactions with people are important. After all, the field which you wish to enter involves very close contact with others.
Good luck!!