Whatever they ask, be confident in your answers. If you don't know how to do something they ask about, respond with "I'll learn it quickly".
Make sure you sit-up straight and make eye contact. Speak slowly (not FOB mode) and enunciate. Communicate your answers quickly and efficiently (ie. don't ramble).
If they ask you questions about your greatest strengths/weaknesses, say "I don't feel there is any one thing that can make-up my strength/weakness. My goal is to make sure that I do the best job I can do here and will move quickly to ensure I head-off any mistakes I could make".
You'll be fine. Just relax and try and have a conversation. They are just trying to get to know you past what is on paper.