I agree with quonnie, a wedding is far too important to mess up. It took about 20-30 weddings before I became comfortable enough to not have nightmares the night before, and this was AFTER I acheived my Associate Degree in Professional Photography! You have to start somewhere, but you MUST understand all the basics of photography (the science) before you can become an artist at it. There is still a lot to learn: posing, lighting, available light, etc...

Don't feel bad! I have an associate degree in Pro Photography and 25+ years experience with medium-format film cameras, and I felt overwhelmed with the complexities of digital SLRs. I found the Magic Lantern Guide's mulitmedia workshop book/dvd for my T3I very helpful. If you have any specific questions, please just ask.

Here are some guidelines I learned over the years: When posing groups, each person should have their own vertical space; arrange so that every face makes a traingle with the closest two faces in the group and keep on doing that until the group is built. For example, if you have parents/grandparents sitting in the middle of the group, make sure they are turned toward each other, and place the smallest child on a lap between (not under/in front of) any one's face. If hands are showing, keep them turned so that only the side of the hand is showing, not the back.
As far as getting the right smile, I have NEVER used 1,2,3; or the word "cheese". You will usually get a "cheesy", forced, unnatural smile.
Try these catch phrases: "Mom is perfect." "My wife/husband is perfect" or "Everyone say thirteen" and shoot during the "teen". These have worked well for me.