Hello, I just recently did my first wedding gig too. I'll give you a bit on my experience.
Mine was in a church, so the lighting was very consistant. There was quite a bit of side lighting from some long windows on the south side, to the grooms right.
I shot in AV mode, which was stupid. I kinda figured out later that I should have simply found a good ambient exposure and set the camera on manual. The problem I had was that any shot taken facing north was great, and all the shots I took facing south were extremely underexposed. this is because the windows were throwing the exposure meter all to hell, even though I had the meter locked to my AF point.
Another thing was that I was ready and waiting for everything to go by so fast... when in fact it seemed like I had all the time in the world... this was a lie. it WAS going by so fast, and I seemed to be missing all the really important stuff. I did get it all, but it was only because I'm quick on the trigger. I did blow the kiss though, 5 shots, all blurred. Not good.
Because your wedding will be with a JP, its going to be even shorter, so make sure your in a good spot when they do the important things, like the giving of the rings, the kiss and the signing.
I shot quite a bit at the reception too, again, this is a great area to have your camera set for manual exposure.
Flash.. if they say you can use it, flash away. Cameras suck in low light, use the flash. I don't know your experience level with flash, but its really easy to make some really ugly pictures using a flash, so make sure that you're on the up and up. This isn't a place you want to be experimenting.
Those are the areas I would have changed personally... Good luck though, an let us know how it went.
Canon camera, Canon lenses.