If you want to become more comfortable with using flash in general, a good site for learning about it is
Strobist. If you follow the Lighting 101 tutorial, and practice a lot with what it teaches, you will become much more comfortable.
One problem, however, is that while getting the flash off camera, as that site recommends, is really great if
possible, it may not
be possible at a wedding, especially if you have limited equipment to work with. Bouncing the flash, though, is often possible, so try to practice that a lot. There will be many times though, that even that is not possible, so you should experiment with diffusion methods as well. Any little bit that you are able to get the flash away from the lens is good, which is another advantage to a flash grip.
The one exception to off camera or indirect flash being better is fill flash for outdoor shots (although off camera flash can still be good for this), so practice with that would come in handy as well.
I have only shot a wedding as an unpaid second shooter (thank goodness). I would not even consider getting paid for it at this point. I'm not sure if I ever would consider it.