I am a volunteer for The Citadel Admissions - in the mid-atlantic, so some advice -
Dress appropriate as someone suggested - business casual but comfortable for you. Most important - ask every question you can think imaginable about the college, the first year, the Forth Class System, and about the academic major you intend to pursue. This is your chance to get one on one with a college staffer who knows the college from the inside - don't fret the interview in terms of what they may ask you. This is YOUR first big decision in life - try not to blow it worrying - ask questions.
One caveat - if the visit and interview is in regards your competition for an academic scholarship or grant, definitely a short resume or list of accomplishments that show your academics and that you are involved outside the classroom - and are not self absorbed with your self is a good idea. If the school invests its limited scholarship money in you - it wants to know you are the kind of person that will make a difference when you graduate.
Good luck!