First, congratulations! Internships are outstanding ways to earn tons of experience.
Second, recognize that you are competing against a pool of very well qualified applicants. Having gone through the match myself oh so many years ago, I remember being accepted into one, meant being rejected from several.
Third, I can't emphasize "succinct" enough. Many resumes are glanced at for only a few seconds, so everything should be very well organized, and relevant. None of this flowery "feel I can contribute valuable blah blah blah to your organization" stuff, just tell them where you've been, what you've done, and in one or two sentences, what you gained from the experience.
I studied here, and got a bunch of A's. I also ran the student society, wrote for the newspaper, worked after hours at McDonalds, etc.
I volunteered here, and learned that while I really enjoy A, B and C, I am less interested in D and am excited about getting better at E.
Paint a vivid, tangible picture of yourself for the reader. What you love, what gets you up in the morning. Make the reader want to Google you to learn more about your accomplishments.
Good luck! And remember, you're still a good person even if you don't match where you want. Happiness isn't getting what you want, it's being happy with what you get. 🙂