Check your camera manual (page 64) for how to adjust exposure compensation manually.
Photographing a wedding is fun and challenging. You'll need to get the white balance right too. I'm not sure how helpful this article will be for taking a wedding outside in the snow, but it may contain a few helpful tips from it:
http://www.digicamhelp.com/learn/nature/snow.phpSome related articles:
Exposure bracketing
http://www.digicamhelp.com/taking-photos/advanced-techniques/bracketing.phpHere are some excellent tips for taking wedding photos
http://www.digicamhelp.com/learn/places-events/wedding.phpWe sure hope all goes well. Don't forget to take some test shots.
