it all depends on the job you're interning for. i would at the very least... go in a shirt (not tee shirt), tie, slacks, socks, shoes. cut your fingernails, cut your hair, take a shower, shave... try to get there at least 15 minutes early.
you see, him, smile, give him a firm handshake while looking at him (face).
when you are there, sit attentively, don't slouch. make eye contact, but don't stare him down or something, when he asks a question, just answer it, don't ramble on and on.
give him a firm handshake while looking at him (face). thank him for his time. ask him how to proceed from there and remember to ask for a business card of his.
send a thank you note to him (email or snail) two to three days after the interview.
good luck dude!
keep in mind that this is the way i do it, with a suit, you don't need to follow everything i do though.