Writing an autobiography is in many ways just like writing a novel. Unless it is just for your own enjoyment, it must be well written and interesting to other people. Whether you are a famous and successful person or just an average human being, you can probably find plenty of interest to write about in your life if you think about it. The keys to writing an autobiography are structure, language and theme. It must be clear and easy to understand, written in an interesting way, and relevant to other people's lives.
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 subsections 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.
Be balanced. Don't just focus on your triumphs -- try to tell people what you're not so proud of and what you don't like about yourself. This will make your book more compelling and also more instructional.
When mentioning other people, try not to mention anything they wouldn't like you to, if possible. Try to not be mean or spiteful. Again, balance is key.