There will be a lot of people who'll say you shouldn't be the main photographer, not as a first timer. Well I feel that as long as the couple knows where you're coming from, you should be fine, main photographer or not. If they don't have a problem, then neither should anyone else. I started my very first two wedding shoots for friends as the main photographer. I wasn't that great, but there wasn't a problem. So don't worry about it.
As far as settings go, the safest way is to stick camera to Program mode on camera, TTL mode on flash and let the camera do the work for you. Get an Omn-bounce, do bounce wherever there's a ceiling. You won't score high on the creativity aspect, but like I said, it's safe.
Get an itenary beforehand and study it. Look out for the significant moments. On the day, don't be so chatty until you forget about what's happening around you. Anything you find interesting and/or important, just shoot.
Before event, check all eqpt in working order. All batts charged, CF cards formatted, ready to go.
And most of, enjoy yourself! It isn't as intimidating as people make it out to be.
Good luck, dude.