Smile, keep eye contact, do not try and be funny. Try and talk to a recruiter beforehand, ask them if they have any advice. The PO did when I went for my interview; it was a great help in calming my nerves.
Have another browse through the Interview thread this evening and make a brief note - just a couple of sentences - of what you could say in reply to each question. You can refresh your memory by reading your notes on the way to the interview. Here you go: