The kid is asking a valid question guys, we can give him an honest answer. Geesh - he is like 18, how is he supposed to know about the grown-up world?
TO answer the question, you need a suit. As a VERY young applicant, one of the things an adcom is going to look at very carefully is maturity. A suit shows that you know how to play the game of being an adult.
So what kind of suit?
Well - dark. Preferably dark navy, or even black. The shirt should be conservative (white, or powder blue) and the tie should be conservative as well (red, blue, or yellow with a simple pattern or stripes - no cartoon characters, or anything silly). Your shoes should be dress shoes, and should be polished. If you are wondering what your suit should look like, look at any politician. Yeah - they are ugly, but that is what your suit should look like.
You should also shave, and have a haircut. Take out any earrings or facial jewelry (including tongue rings).
There will be some clown coming behind me and saying "just be yourself, leave your earring in, don't give in to the man... blah blah blah" but I would argue that getting an acceptance is more important than expressing your individuality.
Good luck!