Find the purpose of your story. Begin by identifying the reason you believe your life story will be of enough value that a publisher will want to publish it. The story of someone becoming a superstar in the Olympics isn't going to be high on a publisher's list, but a story about an Olympian who suffered a severe accident as a child and was told they would never walk again is a story a publisher would definitely take a look at.
Eliminate anything that doesn't support the focus of your story. No matter how important or special many of the events in your life are to you, they won't mean anything to the book-buying public. They are important to you because they belong to you. Learn to recognize those events in your life that are of no interest to the audience you want to target for your story. A bunch of childhood memories strung together don't constitute a life story a publisher will want to publish. Write and rewrite until you have a polished draft that presents your life story with the story's purpose intact.
Find a publisher for your story. The Writer's Market published by Writer's Digest books is the best resource for this purpose. You can purchase the book or sign up for the Writer's Digest database online. The online is updated daily, so it's a better option. The Writer's Market lists short story and novel markets, gives you contact information for publishers, tells you the type of manuscript you can submit, how to submit, who to address it to, and what the payment terms will be. Choose a publisher that handles nonfiction life stories that resemble your life story, prepare your submission according to guidelines, and make your submission. Allow the appropriate time frame before you do a follow up on your submission. If you're rejected by one publisher, don't get discouraged. Move on to the next.