While I agree with TR to "be yourself," if "yourself" is ripped jeans, t-shirts with holes in them and flip-flops, I might reevaluate that form of dress. Best advice my son ever got was when he was 14 from a coach who helped set him up with his first showcase...all he said was: "look like a ballplayer." Take it from there.
P.S. Just in case no one has ever mentioned it to you, a firm handshake and looking right at the person you are shaking hands with gives a better first impression than almost anything else. Good luck!