I wish you well, but don't be surprised if your free wedding photography turns into a friend you used to have. I've seen it happen time and time again. They say they have no expectations, but in reality they do.
That being said, and supposing you will do it regardless, there are MANY things you have to know.
1. First of all, you should attend the rehearsal and pay close attention. All churches have rules about photography. Some even have different rules depending on the presiding priest or minister.
2. Go in your bedroom and turn out the lights. Now light a few candles. THIS is the environment you will be shooting in for the entire evening. Now imagine doing that from 500 feet away. This is most likely the senario. Remember, flash is strickly forbidden except for procession and recession.
3. It is considered rude to shoot during prayer. The Eucherist (sp?) and homily are especially touchy. Ask for rules. Photographers have been tossed out of churches for non compliance.
4. Allow yourself 5 minutes per set up regarding formals. The easiest way to do it is to shoot out some before the wedding.
This would include:
Groom
Groom and Best Man
Groom with each of his men by themselves
All the groomsmen by themselves
Groom with all the groomsmen, ushers, and ringbearer.
Groom with above adding ushers.
Women are the same. This should happen BEFORE the wedding.
Afterwards you should do this:
Bride and Groom with ALL OF HIS FAMILY.
Bride and Groom with ALL OF HER FAMILY.
Do this one at a time, taking out aunts and uncles, then grandparents, then siblings, until you are down to just mom and dad.
Then comes the bridal/groomsmen party. You will want:
Bride and Groom with Groomsmen
Bride and Groom with Bridesmaids
Bride and Groom with Entire Party
Bride with Groomsmen
Groom with Bridesmaids.
That's pretty much the manditory family list.
The reception will most likely be darker still.
Take a multitude of detail shots, as this stuff costs way more money than seems sane. Lots of shots of cakes, lots of gifts, lots of table arrangements, the guestbook, on and on and on.
Table shots are another pretty much manditory thing. Go from table to table, and take shots of each couple individually. This can also help in reprints.
Lastly, don't miss a thing or you will hear about it. LOL.
Parting wisdom is to forget zooms, except for 2.8 or lower with stabilization, and try to use lenses 2.8 or faster. Preferably in the 1.4 range if possible. If you are shooting 3.5/5.6 expect to be totally bummed out.
Good luck to you, and I hope this post has helped you understand the wild world of wedding photography.
Let us know how it goes.
All the Best!