2 Answers

Start making notes, and be chronological.  Maybe spread out pages and put a different year on each one.  As you think of things you want to include, make notes on the paper that has the year. 
Are you going to write in first person (I, me, my vs. he, his)?
Are you going to just record events, or do you want to personalize them with little anecdotes about the people in your life, and how events and people affected you?
What underlying message do you want to convey, if any? (Every cloud has a silver lining?)
Do you want to be conversational in your writing, or more formal?  
There may be a place on the internet that can give you more advice.  Type "write my own biography" in the search line and press the enter key.  See where it takes you.
Have fun!  

Just type it into google.