There are many ways to begin a story: one is writing from personal experience. Here’s how you can use this technique.
Ask yourself what moments from your childhood stand out for you? Which ones can you recall in vivid detail, which stories did your parents repeat to you over and over, which ones were the most terrifying or the most exhilarating? When you find a moment with energy, write it down. It may not become a story right away. It may begin as an anecdote. But collect these anecdotes until you begin to find the links and connections that will pull them into a story.
You may want to fictionalize personal experience, combining real events with imagination to make a story. I used this technique in Take the Stairs, which is based on personal experiences I had as a teen as well as those of people around me. Or, you may want to retain the flavour of actual events, using real names and places, like in a memoir. Either way, you’re looking to arrange reality into a story.
Here’s a quote from Alice Munro that I like: “Anecdotes don’t make good stories. Generally I dig down underneath them so far that the story that finally comes out is not what people thought their anecdotes were about.”