Start with your earliest memory - or at least a very early memory.
Experts say people can remember back to about age three. My father, for example, says he remembers saying good bye to his aunt as she left on a trip to New York. He says he was about three years old. I remember being in a large crowd of people and lifted up on someone's shoulders to see what was going on. I was about three years old!
Neither of these events are history-making but they do show how far back our memories take us. We may not recall every detail of what happened or in what order but we do remember the significant parts and those bits, even if they're just flashes of memory, become part of our story.
If you're stuck with where to start your personal history story, try thinking back to an early memory. And take it from there.