Obviously, you’re not being asked to crew on a two-person dinghy. The crew on a boat that small has to know what he (or she) is doing, because there’s not much room for error on a racing dinghy.
So you’re going to be on a bigger boat doing one of two things, mostly: 1) preparing food for the crew in the galley (generally not a particularly onerous task: sandwiches and coffee, mainly – you won’t have the time or the room to prepare ‘meals’, and the crew won’t have time to enjoy them), and 2) sitting on the high side of the boat during beats to windward.
That’ll be pretty much it, I’d expect, until you do know more about what’s required. But that’s okay, because it will give you a chance to see what others do and how the boat is driven.
For starters, here is a video on rigging a sailboat. Check out some of his other very elementary sailing videos, and you’ll get a feel for some of the things that you’ll need to know. (Keep in mind that the Sunfish in the videos has a rig that is not at all like the one you’ll be on, but other than the rig itself, sailing is sailing. And no one is going to expect a beginning sailor to know much about how to rig a racing vessel, anyway. You’re a long way from that!)