The aim of such an essay is obvious: you need it to convince admissions officers to accept you into college. As a rule, there is a word limit specified in the instruction for applicants. Don’t confuse an autobiographical essay with your CV or resume! They have totally different structure and formatting. In addition, the essay is aimed at portraying you as a personality, not as a professional.
Develop a central theme, main idea or thesis that clearly explains the purpose behind your autobiography. For example, your theme might be about how you overcame obstacles to become a stronger person, or it might explain why a particular passion -- such as mountain climbing or rescuing animals -- is so important to you. Back your theme with details and background information about your life experiences. For example, you might explain that your dad is in the military, so you've had to learn how to adapt quickly to new schools and unfamiliar environments. Or, you might discuss what it was like growing up in a poor, single-parent home, but how your family was always willing to take in injured or stray pets.