The beautiful thing about a personal narrative essay is that no one could ever write it but you. That is, it's your story to tell, and it's likely no one else could tell it any better. That being said, the key to narrative writing--and to all writing, for that matter--is choosing the right topic or issue. In this case, the topic is something which happened in your life and which you can now write about with some reflection.
A good place to start is by brainstorming incidents in your life which have had a significant impact on you for any number of reasons. Think about some of the following as ideas which generally work well for a narrative essay:
- Important firsts in your life (first bicycle ride without training wheels, first time you attended a funeral, first time you got caught lying, first time you drove the car on your own, first time you scored in a basketball game...)
- Significant changes in your life (divorce, moving, changing schools, a sibling being born...)
- Moments of revelation (understanding adults in your life are fallible, realizing death is nearer than you imagined, being betrayed by what you thought was a friend...).
Generally speaking, these kinds of moments will be easiest to write about if they are both memorable and meaningful. Also, be sure you haven't just "romanticized" the event--be certain you can recall enough details and insight to make the story meaningful and interesting to others.
Best wishes on your essay!