Make yourself a checklist of must have shots (don't let the couple see this in case you miss some). Use a mild telephoto as much as you can (helps separate focus from background). Bring all your gear. Have spare batteriesand memory cards on hand, formatted, charged, and ready. Go to the store right now and buy a bunch of wedding magazines. And if this is your first wedding, you're not prepared, and you are freaking out, and this was a last minute request by the couple, then make sure they know what you' and they are getting into.

Do your best, and read up on wedding photography a little more before your next shoot.