Assemble photographs and videos of events, family and friends. The photos and videos don't have to have a particular order and they don't have to cover every year, or even every phase, of your life. They serve the purpose of stimulating memories that fit into a personal history. For example, a photo of a college friend with a perfect smile may trigger the memory of the mountain bike trip the two of you took the year before. She hit a rock, flew over the handlebars, broke her arm, and chipped a tooth. The adventure of getting her out of the canyon to a hospital stands out as one of your great stories.
Make a record of the stories as they take shape in your head. If you're keeping an audio record, use a digital recorder and put the stories into different files. Do the same if keeping a written record. Having the stories in separate files labeled with a keyword from the story will help with organization later. Depending on how much fun you want to make the development of the personal history, you could invite a best friend or family members to go through the pictures with you and contribute their memories as well.
Enhance the stories with a little research. Sites such as Ancestry.com provide tools to help dig into family history to add color and depth to your personal history.
Identify your audience. You may intend the personal history to serve as a kind of journal--a record for you alone. You may want it to help family and friends understand you better, or to leave it as a record for the next generation. Another possibility: You aim for a broad audience with nothing more in common than an interest in the events of your life. Your target audience dictates what happens next in terms of organization and packaging.
Organize the stories. A book intended for personal use or family's and friends' interest only will likely take shape chronologically. Sort through the files and put them in order. Whether or not friends or family had any role in the process before, this might be a good time to involve them to get the order correct.
On the other hand, if the intended audience involves a lot of people who don't know you, then chronology may fall short as an organizing principle. A book designed to have broad appeal needs to draw the audience into the person's life quickly, so beginning with an intriguing story tends to achieve that better than something like "I was born in Ohio."
Work with an editor. Even if you have only family and friends in mind as the target audience, bring someone else into the process to prepare the final product. For a personal history meant for those close to you, a sibling or cousin might be a good choice. For a record created to stimulate commercial interest, work with a professional in developing the book--called a memoir or autobiography--unless you have the background in writing that supports completing the project on your own.
The additional value of working with a pro is that the person may provide time-saving guidance in finding an agent to represent the project and forge a deal with a publisher.