I think the first and best step to getting things out – but not necessarily writing your life story for anyone else to read – is to buy a journal. Start writing about your experiences and painful memories. It will hurt, but the only to get past the pain is to go through it. The more it hurts, the better it will be in the long run to process the experience.
Limit it to 15 minutes, no more – at least at first. Fifteen minutes, three times a week, always at the same time and always in the same place. Stay there for all 15 minutes even if you can’t think of anything to write. This will set up a rhythm, in the same way we get hungry at mealtimes whether or not we’re really hungry. This isn’t just about finding time to write your life story.
What's your central conflict? What's the biggest obstacle life presented that took years to overcome or come to terms with? Maybe it's an illness you were diagnosed with at an early age, a relationship wrought with turmoil, a series of career setbacks, a goal you worked for decades to achieve, or any other number of things. Look to your favorite books and movies for more examples of conflicts.
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.