You might write about a turning point in your life--perhaps a time when you were forced suddenly to grow up, a time when you faced a difficult challenge, or a time when you reassessed your values.
A narrative should have a central focus, but it is not always necessary to express the focus in a thesis sentence early in the essay; at times you will want to get right to the action. A narrative should of course be based on personal experience. Aim for an essay from 300 to 500 words long.