It has been said that all good stories begin with "Once upon a time." All good fairy tales certainly do.
But someone's life story, a person's memoir, is not a fairy tale, and while they could start with "Once upon a time ..." personal histories usually begin with events. The usual method is a chronological telling of a person's life, so it begins in childhood. The person telling the story recalls a hometown or neighbourhood, parents, upbringing, playmates or activities and leads on to school days.
Another way is to recall an early memory, where the person was, what was happening, who else was there, why that person remembers the event and so on.
It helps, too, to think about the events before telling. That helps clear the mind and prevents skipping important bits that might otherwise slip by. It helps, too, to tell the story to someone else who can help jog the memory with a few gentle questions. Personal historians are trained to do that. In fact, contacting one is a really good place to start.