1. For a wedding reception, don't bring a tripod....things are too spontanious for that.
2. Buy or rent a 580ex flash.
3. Spend $25 for an L flash frame, to mount the flash off-center and high above the camera.
4. Get an Omni-bounce diffuser (it's cheap $20).
5. Rent/buy these cheapo fast primes: 50mm F1.8 or F1.4, and 28mm F1.8. 50 is too long in reach for some shots...that's where the 28mm comes in.
6. Rent/buy this: Canon 24-70L F2.8. use this for most shots, but the fast primes for those shots with more challanging light issues.
7. Shoot only in raw. Flash all the images. Leave the available light for more seasoned shooters.
8. When shooting more then one person, be mindful of the required depth-of-field and you control this with (1) distance from subject, (2) apertuer setting (F-stop), and (3) focal length.
9. Manually pick and choose the appropriate focus point, and try not to recompose after AF lock.
10. Rent/borrow a back up camera body and flash too.
11. Built in flash sucks...you will be sorry if you use it.
Ok, now I suggest fast glass so that your camera can acquire focus lock faster and easier then if you used a slow lens. This is a valid for reason fast glass even if you never need faster then F4....but you will.
If you have time to learn, google "dragging the shutter". If you don't have time shoot in P mode, but the pictures will be so-so. Learn how to shoot with the body in manual, flash in full auto so that it takes advantage of E-TTL.
There is so much more I didn't mention here for lack of time....but this is a start....will there be a pro there as the primary shooter? I hope so, for the sake of the bride & groom.