You saysomething about turtles... No wait, wrong smalltalk.
Seriously though, small talk is fleeting. You pick a generic subject (Something like: what's up? How are you doing? (if the person is doing something interesting) What are you doing? How does it work? Why are you doing that?). Just ask questions, don't get too personal at first. Pick up on clues, ask about what you hear. Keep it light. Backchannel a bit ("oh, I see", "uh-huh", nod, etc), but don't overdo it.
Oh, and practice, preferably with people that you know well.