Although the colleges are pointing you toward big topics, asking you for a significant event or an issue of global importance, it's best to think small.
I blame the colleges a little for their wording. By using the word "significant," they suggest that there are subjects in everyone's life that fall into a sort of MAJOR EVENT category. And probably, at the end of your life, you'll think there were two or three of those. But at 17--give me a break. I really hope nothing profound, devastating, or horrific has happened to you. But I've talked to kids who actually thought it would have been a little more helpful if the parents had gotten a divorce--that's the least they could do to provide good material. In fact, sometimes writers are tempted to "borrrow suffering" for this topic and end up writing about someone they know who knew someone who's uncle died in the World Trade Center collapse. Or a friend who's Mom is now tragically ill.
Don't do this. The significance is something you add, not something inherent in the event. We all know a look, an hour, a pair of green sneakers, a forgotten photo, a word from a great teacher, a new book can have a big impact.
Once you've decided what you want to convey about yourself, think small and make the event important by what you tell us about it, not by the fact it ought to have qualified for Ripley's Believe It or Not. Think small....and make that small event significant. Otherwise, you'll have to start searching through your life for tragedy.