If you do use an external mic, be very careful when you have the camera on AC power. You could end up with a ground loop and have a very loud hum recorded over all your audio.
Use a tripod if you can. When hand-holding the camera do your best to keep it still, brace yourself against a wall or something. Keep your arm against your body to add stability.
Don't go zoom happy. This is probably the #1 home video mistake.
Being that you will edit this footage later, you should work to frame shots knowing that you can chop out the zoom or quick pan that you made.
Make your pans/movements, slow and steady but not too slow.
Definetly interview the guests during reception.
Make sure you get the vows, the introduction of new couple, first dance, cutting the cake, and any of the other standard wedding stuff. Don't be hitting on that bridesmaid during the cake cutting, etc.
Get some establishing shots. Get there 10 minutes early and get the front of the church, empty church, the reception hall, limos, flowers, centerpieces on the tables, etc. These are nice for cuts and transistions.
Good luck and have fun!