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.
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.
“I was born in 19XX year, I entered school N…” Would you read that if you had a choice? Most likely not, especially if you already have read a dozen biographies which star with the same phrase. Yes, an autobiography is a formal piece of writing. But here, it rather means that you are not allowed to use slang and colloquial language, instead of following a specific pattern of writing.

Use chapter divisions to give the book structure. Chapters are useful because they allow you to move on from discussing certain life periods or events. There's a reason we have the expression that we've "closed a chapter" or "opened a new chapter" in life, and it's even more applicable when discussing an autobiography. Chapter breaks allow you to skip forward 10 years, go back in time, or start describing a new theme without jarring the reader too much.
If writing skills don’t come naturally to you, these skills can be acquired when you take a course to learn the basic skills of writing. Such course help you by guiding you to choose the style of writing your autobiography that best suits your personality. This style will then create a powerful impact on the reader and their response to your outpourings.
Create an outline or a graphic organizer of the major topics and ideas you want to cover. For example, you might make a time-order chart -- a chart with event blocks to organize events chronologically. Or, you might draw a cluster web -- a diagram with connected circles -- to divide your autobiography into sections, such as one for relevant background information, one for life-changing events, one for special memories and one for goals and dreams. Focus on reasons why specific events, situations and circumstances were important to you and how they made you feel.