An autobiographical narrative is simply a story written about an event the author's life. Think of a time that something interesting happened to you that you might like to write about. You will naturally have to include descriptions of the event to help your audience understand your experience. For example, I might write about the time a tornado destroyed my house. I would have to describe what the house and the neighborhood looked like before the event so my reader would understand the extent of the damage. I would also want to describe the tornado itself and how it felt as it passed through our house while we were inside. I might also have to describe my family so the reader would understand my concern for their safety. I'll give a more specific example. Without explaining that my mother had just left to drive to church when the tornado hit, it would be difficult for the reader to understand how confused we were because we didn't know whether she had made it or not.
Any narrative should follow a story line and autobiographical narratives are no different. Pick an event in your life that you would like to write about. Describe the moments leading up to this event, the event itself, and the moments following the event. I think you will find this type of non-fiction writing to be fairly straight forward.