Autobiography has many purposes from telling your story as a gift from one generation to another to serving as a powerful tool for reviewing your life to build self-awareness, self-acceptance and increase life satisfaction.
It is similar to journaling but more focused and usually told in chronological order. There are lots of great resources on the web that give more direction.
As you write don't worry about editing unless you hear yourself including an untruth then explain what it was, why it was there and what it told you about yourself. Then continue writing from there.
Be cautious though if you decide to share any part of it with others. Be sure to tell the person in advance if you want any feedback or if you just want someone to listen. In reading it you might recognize an area of new insight or an area that is ready for additional healing.
Writing autobiography the first time for me in my dissertation was to build understanding of my life and my resilience. The second time I wrote it with additional detail for my book Living Hope turned out to be all about emotional healing.
Consider: Write a bit of autobiography starting at any point, ending at any point. Is there some part of your life that to share it would be valuable for you to explore, deepen your understanding or may be healing for you? You don't have to write your whole life. Think of writing excerpts, or the parts you want talk about. Spend an hour or hours on it.