Start small. Make some notes on a large piece of paper about the big events and phases in your life, such as your schooling, college, first job, relationships and wedding. You could even construct a timeline. Include major world incidents as well, so you can relate them to your story.
Think of several short, easily digestible stories from your life you can tell in your book. They don't have to sum up your life all in one go, but anecdotes that demonstrate who you are or how you see the world are excellent fodder for an autobiography. They can enlighten the reader much more easily than just describing your qualities.
Split your life up into three to five major sections. These can be based around places you've lived, relationships, jobs or any logical division that you choose. Each section should start and end with a big turning point in your life. Order these sections chronologically, and they can form the framework of your book.
Think about what you want to say. It's important to take time before you write to decide what lessons you want people to take away from your book. What has your life taught you? What advice would you give to people like you? What would you do differently? You won't be answering these questions directly, but your whole autobiography will tell readers about your attitude to these issues.
Write. Start with your childhood or with some particularly vivid memory. Your three to five main sections will break down into the sub-sections that will form chapters. Write as though you were just talking to someone, telling the story of your life. Your voice should come through in your writing, as this will help reveal your attitudes. Don't try too hard when writing -- just enjoy it.