Each writer has a different tactic on the best way to begin a project. Some like to brainstorm and organize their thoughts about the entire project before beginning. Some writers find it helpful to create a time-line for the project and break the writing tasks into sections with separate deadlines. Others just start writing. Many people begin their projects writing in notebooks or journals. Others start blogs or electronic journals. Organize and write whatever way is comfortable for you.
Read examples of autobiographies before and/or while you work on your own. Seeing how other writers have tackled a similar task can be greatly inspiring. A few examples include "Woman Warrior" by Maxine Hong Kingston, "Black Boy" by Richard Wright, "The Glass Castle" by Jeannette Walls and "Maus" by Art Speigelman.
Many writers find the use of writing prompts and textbooks helpful when starting a new writing project. Good textbooks with both prompts and reading material include "Metro: Journeys in Writing Creatively" and "Creative Nonfiction: A Guide and Anthology."
There are numerous writing prompts geared specifically toward autobiographical writing. Some can be found in writing workbooks, textbooks and online. Writing prompts cannot only get you started each time you write, they can bring about the beginning of an entire book project.
If you can write tenaciously and with a routine schedule, you're on your way to being successful. It is best to set aside a certain amount of time each day (or every other day) for writing or working on the project. Many writers find it easier to schedule a time for writing at the beginning or end of the day. Whatever time you choose, it is best to make it routine.
There is no rule for how you go about writing your autobiography. Some people write a collection of short stories where each piece has very little to do with the other pieces. Others start with their earliest memories and continue to write in a basic chronological order. Many autobiographies only talk about a span of a few years. There are also autobiographies that discuss a single topic--such as music, work, relationships, sports or art. Some discuss family relationships and childhood memories in detail. The possibilities for your project are limitless.
Feedback is an important aspect to the revision process. You can find helpful information about your writings from having an informed audience read your work. That is one of the reasons many people find taking a writing course helps strengthen their work. Community colleges, universities and community centers offer writing courses that focus on creative nonfiction writing or autobiography specific courses. Other than giving you an audience, writing courses can give you a scheduled time each week to focus on your work and an deadline to have a certain amount of work completed (classroom assignments are great for that).
A similar way to get feedback and a deadline for work to be completed is by joining a writing group. There are writing groups who interact entirely online and those that meet locally to discuss their member's work. Writing groups are usually free, and there isn't the pressure of required reading lists and graded assignments. Both writing courses and groups can be useful to writers, but ultimately it's your decision whether to utilize them.