Good question, Kaella. The method you will use depends heavily on whether you will be recounting a true story or creating a work of fiction.
If recounting a true story, the first thing you should do is freewrite everything that you can remember about the incident, not just what happened but all of the sensory details -- what you heard, saw, felt, smelled, and tasted. Also jot down some details about the personalities and physical attributes of each of your friends.
If creating a work of fiction, you will have to do something similar, because you will need to create characters and setting by supplying those kinds of details. But there you have more latitude. You'll be using your imagination rather than your memory. You can choose to make the story tragic, comic, or anything in between.
Either way, in thinking about the plot of the story -- what happened -- ask yourself what is the climax (the most dramatic part) and how can you build up to that in a way that creates tension and interest in the reader. I assume that the climax will be the falling in, although it might be the rescue if that was difficult. Is the one who fell in a reckless person? Can that be shown right away, in something that you say about him or her or some dialogue? Earlier in the day, did one of the other friends warn that one not to be reckless about something else, thereby foreshadowing what was to come? These are the kinds of details to include to create dramatic tension. To bring the reader into the story, use lots of sensory details such as the sound of the falls or the feel of its mist.
Simone, EssayForum.com