I come from one of the most penny pincher families I've ever heard of and I totally inherited it. So If you are looking at prices and saying I bet I could find someone cheaper I don't blame you. I look at my own prices and roll my eyes sometimes but the bottom line is I love shooting weddings and families and everything in between and in order to be able to do that like I want I have to charge. It's my least favorite part of the job. However I'm also flexible because my favorite part of the job is delivering good pictures and I think everyone should have amazing images to remember memories. So I am willing to be begged, bartered with, and bribed into taking your photo's because I think they are important.
I also know that as soon as you get engaged ever relative and friend with a nice camera offers to take your pictures. And I know they mean well and I know they want to help and some of them are good. But as a professional we do it more often have had more training and this is perhaps the most important and when something goes wrong we know what to do. If you get into the reception and the lights are low we have the gear to make up for that. If our camera malfunctions we carry a back up. The same can be said for most of our equipment. Your friend may have a nice camera but if that one drops are you left with whatever they can get with their camera phone of your special day? And is it worth the risk? Also do they know how to shoot in a darks gymnasium, in the pouring rain, at sunset, by moonlight. Do they know how to adjust for indoor light vs tungsten or florescence? If you aren't sure how they will do under that sort of pressure call up a professional and talk through your money concerns and see if there's something that can be done so that you get the best pictures possible to remember your day.
If you still want to use a friend have them check out a few of these tutorials to make sure that you are getting the best images possible. These aren't necessarily my favorite or the best because there's hours and hours of videos out there but they'll be a good place to start.
Understanding ISO: http://youtu.be/gIQpk8xHwjI
Understanding white balance: http://youtu.be/fgyjxQxX_GI
Rule of thirds and composition: http://youtu.be/PF4h9vhtSGA
Understanding f-stops: http://youtu.be/KmNIouLByJQ
Understanding f-stops and shutter speeds: http://youtu.be/kD8nXGt91yo
General settings: http://youtu.be/ap7qnQHIuXI