Hi Amy, Good question. While I can't personally speak to the Ivies in particular, here are some things that most colleges and universities (including, to the best of my knowledge, the Ivies) look for. They want to see someone who can do well in a rigorous academic setting, while also adding something to the community. With respect to the prior, make sure that your grades are good. They don't need to be perfect, but good enough. How good depends on your strengths in the second. If you haven't already, get involved in extracurriculars, be it student government, a sports team, an academic team, volunteering, or whatever else piques your interest. There's an age-old debate over quality vs. quantity, which you'll have to figure out for yourself based on your interests. In all of this, make sure you can explain how you stand out from all the other applicants.
One last thing. Please, please, please carefully consider why you are choosing the Ivy League specifically. It is a popular household name, but it is not for everyone. Many people (unfortunately I cannot think of examples right now) go to them and end up miserable because it's not what they really wanted, they only went because they felt pressured to or was attracted to the name. So ask yourself what it is you really want, and whether the Ivy League is really the best to be looking at. For example, recently, Ronald Nelson turned down every Ivy League school in favor of the University of Alabama because he would have accumulated a lot of student debt by attending an Ivy League school, whereas the University of Alabama gave him a full scholarship (here's the article I found if you're interested: http://www.businessinsider.com/ronald-nelson-turned-down-every-ivy-league-school-for-university-of-alabama-2015-5)
Best of luck, and let me know if you have any more questions!