Do your research. Read other autobiographies and make a note of which ones you found interesting. See if you can pinpoint techniques that the authors used to get your attention and to keep you interested.
Decide on a theme for your book. Most interesting autobiographies have a specific focus--the subject was a famous sports player or musician or actor, or he lived in an unusual place or had an interesting career. Think of a similar hook with which you could whet the reader's appetite. Note down the details of your own life that will make it interesting to somebody who doesn't know you.
Make a list of events in your life that fit your general theme, and decide which of these you can include in the book. Most autobiographies are told in chronological order--you can use your chosen events to work out a rough structure for your story. You can start with your birth, but it's often more interesting to start with an event that captures the spirit of the book--the first race you won, the first time you nailed an audition--and work from there
Do more research! This time, you're looking to flesh out the bare bones of your outline, so pull out your old records, documents, letters and photographs--anything related to the events you've decided to mention in the book. The more individual or unusual details you can include, the better, so that your story stands out from the crowd.
Write! You know what your focus is, and you've got your material, so get writing. Remember, writing is difficult, and you might get discouraged at how long it takes. Don't worry--be patient and write a little at a time. Don't try to do it all at once.
Consider hiring a ghostwriter to help if you've got plenty to say, but you're struggling with the prose. This is a professional writer whom you hire to transcribe your story--it's still your autobiography, and your name will still go on the front cover.