Make sure you have a comfortable place to write before you sit down to write. Have plenty of supplies. You will need either a computer or plenty of paper and a pen or pencil.
Make sure to write down any memory that comes to you after you decide to begin writing. Don't try to write it in a sequence, however. Just write down what you need. Keep a pad of paper and a writing utensil with you at all times. You never know what will trigger a memory.
A stereo and a few CD's of favorite songs could help the writing process. Music helps people remember things they have forgotten.
Speak with a lawyer before beginning your autobiography if you have any fears about being sued. They can help you determine if there are any facts that may be considered libelous.
Be prepared to be honest about your actions, thoughts, feelings and anything that happened to you or within your family.
Ask parents, siblings, grandparents or other siblings what they remember about you.
Ask teachers or other adults involved in your life what they remember about you.
Look at photographs to help you remember events and other things.
Get a couple of books designed to help you remember. Your local library should have some suggestions, but a favorite is "To Our Children's Children: Preserving Family Histories for Generations to Come" by Bob Greene and D.G. Fulford.
Read books about history during the years you plan to write about. This may help you to remember things important to your family that you forgot, or understand things that happened when you were too young to know about them.
Read other autobiographies so you will have a general idea of how you would like to arrange your autobiography.