I recently read a small book espousing a start entirely different from what I have suggested, so here for your consideration, are ideas from Immortalize Your Life by Marc Van Buskirk. He recommends starting with your earliest memories and writing, decade by decade, the highlights of each ten years. Throughout the (brief) book, the author gives illustrations from his own book and suggests that you duplicate them. “All you have to do is change the time, places, people, things, and events.”
By all means use this approach if it works for you. There is no one way to start writing about your life. Based on the experience of helping my parents, I suggest starting with your most vivid memory, the story you tell most often, the recollection that haunts you, the happiest or saddest or most traumatic time. As I have written elsewhere, my father got hung up starting his story exactly right and wrote that beginning over and over.
The important thing is START!