If your writing is atrocious, or if you just need some help getting your thoughts in order, consider hiring a ghostwriter or a professional personal historian. Celebrities do it all the time. There is also a software that allows you to type your answers into a template on your computer, thus solving the problem of less-than-perfect handwriting. Many people also choose to type directly into an online template.
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.