Step-by-step explanation:
You want to focus on a chronological description of your life. From you as a child to you now. Make sure to include your mistakes in life and the times when things were hard for you, it makes you seem relatable. Include your triumphs and achievements as well. Describe you as a person, your interests, beliefs, and morals. Reference other peoples autobiographies, take formats and writing styles you like from there, and incorporate it into yours.
Also, it really helps to make an outline,
1. Childhood
a. Family life
b. memories
2. Teenager
a. memories
b. relationships (optional)
c. hardships and mistakes (you can include this here or in your adulthood)
3. Adulthood
a. What you are doing now (interests)
b. relationships (optional)
c. hardships and mistakes (you can include this here or in your teenager years)
d. how you have fixed or are fixing those mistakes and healing from hardships.
This is kind of the outline you might want to follow ^
REMEMBER this is about you written by you, so you don't have to include what you don't want to.
I hope this helps.