Corny and oh so predictable but - be yourself. If in doubt, plan a few lines in your head first, and have a couple of subjects ready to talk about (not football!) in case you freeze. Yellowduck is spot on.
Suitable topics - music, films, books, mutual friends, school / college, jobs, hobbies, etc. But don't monopolise the conversation.
If you really don't know what to say, ask her questions - most women (and I include myself here!) love to talk about themselves! But if you do this - listen to what she says, don't just block it out.
You'll be fine. And I agree with yellowduck - I love shy guys. They aren't pushy, and aren't usually eyeing up someone else while they talk.