"Narrative" is just a fancy word for story. A narrative essay is usually a true story that makes a point. Unlike a story written for entertainment, a narrative essay usually contains a thesis statement that states the point outright.
Your first job, as you approach this assignment, is to choose a topic. Your best bet is to choose a topic that is familiar to you, a story from your own life, or maybe from a family member's life (with the family member's permission, of course). When I taught students to write this kind of essay, I often started by asking students to write about a lesson they learned the hard way. That gave them an opportunity to tell a story that had an easy-to-find point: the lesson they took away from it.
For other good narrative essay topics, think back to times when you have made big decisions or realizations. Each of these has an inherent point or two that you could make in an essay. Was there a moment when you learned not to trust a certain kind of friend, or when you learned how to be a better friend than you had been before? Was there an experience that made your realize a strict parent or teacher was really acting in your best interests? How did you discover your biggest interest or talent?
Once you have a topic, your next job is to write the thesis statement. eNotes has a great page on thesis statements (link below). In a narrative essay, your thesis should make a generalization that the reader can take away, to give him or her a greater understanding of the world. If you were writing about a lesson you learned the hard way, your thesis would say what you learned, not just what you did wrong.
Good thesis: I learned that robbing a bank has serious consequences.
Bad thesis: A lesson I learned the hard way was robbing a bank.
You see how the first makes a point the reader can take away, whereas the second just states the topic of the story?
A narrative thesis should make a generalization but should not go overboard with that generalization. In other words, you need to fit your idea to the patterns that exist in the world, not make stereotypes out of the patterns you see:
Good thesis: Sometimes it is important to follow a dream, even when parents and other adults stand in the way.
Bad thesis: Parents and other adults always try to stop kids from following their dreams.
Both of these statements make generalizations, but the bad one is a stereotype. Watch out for words like "always" and "never" in your thesis statement. They often appear when you go overboard with your generalization.
Best of luck with your narrative essay!