You're probably more aware of inciting incidents than you think. Whenever you start talking with your friends about any type of storyline — a book, a movie, or even an episode of your favorite television show — inevitably someone will start analyzing what led up to it. What was it that put everything in motion? What's the first important thing that really pulls you into the story? In other words, what was the inciting incident?
Your teacher is asking you to do the same kind of detective work in thinking about Lord of the Flies. Even if you've only read part of the novel so far, you can probably already tell that things go from bad to worse pretty quickly. So ask yourself, what happens that sets this story in motion? What happens to give you your first clue about how the story is going to unfold? When you figure that out, you'll have discovered the inciting incident.
One word of caution: Don't get confused and end up looking for the climax of the story instead of the inciting incident. The climax is the big crucial moment toward the end of a story, the turning point where the major issue of the plot is finally resolved. (In an action movie, for instance, the climax is usually a big fight scene where the hero goes head-to-head with the villain.) The inciting incident, on the other hand, is the moment from the beginning of the novel (perhaps an argument between the hero and the villain) where the main storyline gets cooking.