I'm not too experienced with night wedding situations (or with Canon gear at all), so keep that in mind... I think shooting without a flash is really risky and not likely to work out well. Your ISO may wind up really high and introduce noise into the images, especially if they're underexposed. The light will be uneven and you won't get good light into the faces. The white balance can wind up all over the map, making for funky skin tones. Shooting with your 18-55 zoom will allow you a lot of framing flexibility as you'll probably be on the move. Favor standing back a bit and shooting towards the long end of the lens. At the short end, people can get distorted.
If you shoot with flash, set your white balance to flash and the colors should be right. Get a diffuser for your flash to soften the light and scatter it around. Aim it towards the ceiling if there is one to bounce the light. There's several choices, so maybe consult with a camera store. I use one like this and don't think you can go wrong with it:
http://www.bhphotovideo.com/c/product/407175-REG/Sto_Fen_OM_EW_OM_EW_Omni_Bounce_for_Canon.html
Shoot RAW + JPEG. You can correct for mistakes better from RAW, especially missed white balance. Memory is cheap- it's not like film. Shoot 1-2 backups of every shot to avoid closed eyes and all the other things that happen. If you're shooting people in daylight, shoot continuous high speed 2-3 frames.
Bring extra batteries for your flash. Your filter should be a circular polarizer for a digital camera. Hoyas tend to be a good value. Always have your lens hood on your lens- it helps to protect the lens from physical impact and it blocks out stray light. Shooting into the sun, see if glare is blocked with you hand as well. Practice some outdoor night shots before you leave. Try not to use the built-in flash. Learn how to use the flash compensation to back off the strength so it functions as a fill light. This also allows more correct exposures of backgrounds instead of letting them go dark. Use your camera's automatic mode when shooting flash at night. Lots of weird things can happen otherwise. See if you can find a 3rd party guide to your camera, like Magic Lantern. Factory user manuals are very dry. Get a couple of decent 8GB memory cards. SanDisk or Lexar won't let you down. Get a card reader to transfer to your computer. Only format the cards in the camera itself. Re-format the cards instead of deleting the images after you've downloaded the images to your computer.
Focus-wise, I'd suggest using the center focus spot on the part of the image you want to be in focus, push the shutter down half way to lock, then re-compose the shot. Your lens is going to be sharpest around f/8-11. You can isolate your subject better by shooting "wide open," at f/3.5 for example. The depth of field that's in focus will be narrower, yielding a pleasing, more blurry background. When you want everything in focus, shoot f/11-f/16. Read up on the
Rule of Thirds. Watch out for and avoid distracting elements in the background, especially things coming out of people's heads.
Hope this helps.