It's a good flash so that's great.
As has been said, play around with it at home. I would go for a manual exposure for all but the flash when indoors and have it so that you are about 1 stop below ambient. To do this see what shutter speed but using aperture priority mode and dialing in f5.6 (assuming you didn't change from the kit lens) and ISO 1600 (perfectly fine to use). Lets assume you get 1/20s, That being the case then go with manual exposure f5.6 ISO 1600 and 1/40s. This is quite slow so don't shoot when people are running about, jumping up and down etc, but as it's a wedding you should be fine.
If you found the shutter speed to be much lower than 1/20 with ambient then push to ISO 3200 but I think you will find that you are fine in most churches. If it is daytime then you might only need ISO 800 and get a faster shutter speed, if so this is great. Oh, get to the church early and shoot family members to make sure that you know the settings for when they B&G arrive as you will likely be outside prior to this so will have used other settings. Another thing, in the church use the flash with the catchlight card out and the flash at 45 degrees, this will give a bit more of a splash or flash then over powering. I do this outdoors too, even with no ceiling as long as I'm not over too much of a distance.
The golden rule when we have good light is naturally light first, then reflected then flash. There is nothing worse than shots the are just totally flashed.
If you are doing outdoors shots in daylight and want some fill flash then use aperture priority and I would go with f6.3 which will give enough DOF for groups on the 18-55 lens. Set an ISO that will give you a shutter speed of less than 1/200s and more than 1/60s, this is a good balance with keeping within the shutter sync speed and not getting camera shake. If you have very bright sun you might find that at f6.3 you can't get the shutter speed at 1/200s (you will see all your shots over exposed), in which case for ease go to shutter priority and then dial in 1/200s. The camera will then stop the lens down more.
Again, don't flash unless you need to, but if the sun is high and you have nasty shadows it is worth it.
At this wedding, I was allowed to use flash but to get a better feel of the church and also to be less distracting I chose not to use it apart from the signing of the register. http://www.youtube.com/watch?v=TzxG1F98ajE
It might give you some shot ideas too although it is just a short clip telling a quick overview of the day.
White balance, shoot flash white balance all day, unless you go for no flash in church in which case use the best option for the church lights. Even if you are shooting no flash outdoors use the flash white balance, it will give a nice film feel to the shots, also it will give you colour consistency which is important. Shoot RAW, this will eat memory but worth it when you are trying to sort out colours in the church and also if you blow the exposure at some points you can get it back more easily. Shooting with the 5DmkII and 7D at a full wedding (church, reception etc) I can get through 30-40Gb of memory.
Batteries have been mentioned but worth mentioning again for not only the flash but also the camera, you might need 2 batteries depending on how much you shoot, I've never used my 500D for a wedding to know how many shots I get.
I think that's it for now.
Oh, cover the basic shots first then be creative if you have time.