If possible, get or draw up a floor plan of the location of the ceremony. It will help you plan your camera placement and shots. Position your microphones where they'll do the most good. If possible, get either the bride or the groom to wear one, and get the minister or celebrant to wear one. This way, you'll have crisp, clear sound.
Technical skills are important, but they can be learned.
The most crucial skills are smooth panning and zooming. People look around when they're at a wedding, and so should the camera, within reason. Practice slow, smooth pans that keep the subject in the frame of the camera. Practice smooth zooms that aren't jerky, and aren't too close or too far from the subject. You've probably figured that you'll need a helper for the actual taping.