Amanda, the best advice I can give you (in addition to all the advice already given), is to go to the church with your equipment, and actually take pictures. In fact, take your friend with you and let her pose in various places. You will need to use the widest open aperture of your lens, and at least an ISO setting of around 800. Try a shutter speed of about 1/30sec to start with, and see if the images you are getting are blurred (due to hand shake or camera motion), in which case you will need to increase your shutter speed. Take a whole bunch, take them home, download them onto your computer, and look them over WITH your friend. If the two of you decide the result is acceptable, then both you and your friend know what you're getting into and what kind of results you will get. If they are not, then at least you have given your friend the heads-up BEFORE the wedding.
I've shot weddings as a family member with a camera, using a 24-105 f/4 with a T1i. The f/4 is rather limiting in dark areas, but I was able to get decent (but not very good) shots that were acceptable with a little post-processing tweaking. In all cases, there were professional photographers doing their thing, and using equipment like MLeek mentioned. In some venues, flash was allowed, in others, it was strictly ambient light only.