So here is a quick run down of things I learned at my first wedding. These are good rules whether you're doing it for free for a friend/ family member, or whether it's someone you've just booked.
1. Make up a basic contract. This just lets people know what you're offering and the time limits you're working with, that way they know what to expect.
2. Have a shot list. This can be something you come up with and go over with the bride, or that you have the bride make. Either way this list will insure that you get the photos that are most important to the happy couple.
3. Lots-o-Memory. You can never have too much memory, that way you don't get to the middle of the ceremony and have to start deleting photos.
4. Take your time. This is hardest for me to remember when I'm doing group shots. Sure you don't want to have the posed photos take 2 hours while everyone is waiting at the reception. But remember that this is a happy day and that most of the time, these people have been rushing to this point all morning. Take your time and rearrange people until it looks right to you.
5. Jump in there. This sounds dumb to say, but don't be timid! Sometimes when you're first starting out you don't want to get in the way of anyone and ruin the wedding, which is true, you want to be respectful. But whether you're doing this for fun or for coinage, they picked you to be their photographer! So have fun and jump in there to get the good shots.