Identify the main characters. Every good story has interesting characters, friends and foes who help move the plot along. Who are the characters in your life? It's a given that your parents will play a role, along with your spouse and other close family members. Think beyond your immediate family to others who have affected your life and should play a role in your autobiography.
If the purpose of your autobiography is to pass on your story to your heirs, consider including memorabilia (e.g. pictures, heirlooms, medals, mementos, letters, etc.) and putting your story in a scrapbook format. Of course, you may not be able to copy the memorabilia that accompanies your autobiography, so you still have to think about what you intend to do with your original work and other items, such as medals or bulky heirlooms.
Write a rough first draft. You may feel pressure to write and rewrite every sentence you put down. But part of writing a memoir is writing an honest account of a pivotal event, in your own words and with your own style. Avoid putting on a “writer” voice. Instead, don’t be afraid to write like you speak or talk. Include slang and any regional vernacular. Make your story sound like it is coming directly from you.
What else happened after each of these main events? What was important to you at this time? Did you have any special friends? A pet? A romantic partner? A spouse? What specific event will best show the relationship you had with this person or animal? Was the best friend who helped you through a difficult time a kind and gentle soul? What single act of kindness most stands out when you think of this person? Write about that. What actions and dialogue can you relate in concrete detail to help readers understand that relationship and its importance?
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.