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.
The childhood story. Whether your childhood was happy or traumatic, you should include a few anecdotes that give a picture of who you were and what you experienced at the time. You can tell the story of your childhood by breaking it down into smaller anecdotes that illustrate your personality - your parents' reaction when you brought home a stray dog, the time you climbed out of the window at school and ran away for 3 days, your friendly relationship with a homeless person living in the woods . . . get creative.