Stick to informal language, unless it’s absolutely necessary to use formal terms. Instead of “utilize” you can just put “use”. Focus on simple language, with one or two syllable words. The only time you should be using higher level language is if you are using scientific terms or describing a technical process. Even then, you should be writing for the average reader.
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.