Mmm. Even if you're not confident, act confident.
It works wonders in the way you appear to them. And don't forget to make eye contact - not necessarily I'm-going-to-kill-you eye contact, but just so that you're not looking down and stuff. Your voice doesn't across so well when that happens, and also reduces the impact your words has. Smiling a bit helps, too, just generally being enthusiastic about what you're considering going into (but not too much, or it's overwhelming.)
Make sure you know what you wrote in your application for co op - that'll almost always come up. (which co op are you going for, by the way?)