Morgan, don't feel pressured to come up with a "significant" or "life-changing" event; not everyone has had one of those by age 17!
An essay can say something about you without being direct. Every creative, interesting essay has a subtext - intended or not. Readers will analyze you from what you say and how you say it, much the way you would do about a character in literature. My advice is to look for your topic first, and then see what that says about you. If you don't like the message you are broadcasting, try again.
For example: An essay about a tough class you had and how you eventually got through it might convey to readers that you don't give up, are an innovative thinker, etc. An essay about what your desk looks like right now would probably convey a lot about you - this person is organized and focused, or has eclectic interests, or gets excited and carried away by his/her passion for projects.
Make sure your essay doesn't convey the wrong message. Writing about not doing well in a class because you had a bad teacher, for example. Or about your triumph on the football field or in the orchestra (nobody likes someone who is really full of themselves, right?). In fact, some of the best, most likable, and entertaining essays are about mistakes!
I hope this helps. Good luck!