People choose to write about their lives for a variety of reasons, including a desire to leave a memoir for their children and future generations, to create a record for themselves so they can be reminded of their youthful adventures when they're old and forgetful, and to offer something of value to the rest of the world. Writing a memoir is a very personal experience, but if you're willing to share your life story, it can be incredibly rewarding.
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.
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.