Just some things to add:-
Construct a shooting plan prior to the event with the bride and groom this gives you something to work from and also makes them think about what shots they want and who they want in it. You can also add your own creative idea on to that as well.
Plan the important shots and take them safely i.e. go for settings you know will work once you have the key shot then play around with cretivity.
Try to do a pre wedding shoot this will help the bride and groom to get more relaxed with the camera and give you an opportunity to get them to pose sensibly.
To get more natural shots be as unobtrusive as possible, even if you are allowed flash during the ceremony I would avoid using it as it interrupts the event you are there to capture, key moments such as signing the register, exchanging rings, saying vows can be faked as well after the main ceromony. Also worth faking the cutting of the cake.
Try and get a copy of
this book it has helped me a great deal.
If they have said they will pay you something agree a fee up front even if you are not going to accept any money from them, this will make them take your role more seriously and get you more cooperation on the day, this may sound a little harsh but you will need it.
Have plenty of spare batteries and memory cards, also have a backup plan in the event of equipment failiure even if it is a compact.
Watch the light, should not be an issue this time of year but I shot a wedding in October, the main ceremony finished at 4:00 PM I was shooting in AV and didn't notice the light levels dropping slowly as a result a couple of shots were not as sharp as they could have been.
Equiment wise I used a 10-20, 24-70, 70-300 most shots I took were with the 24-70 with fill flash. A tripod is handy as are external flash and reflectors.
Look at other wedding shots for inspiration.
As already mentioned try and find out what they want on couple told me they just didn't want shots with red eye or things sticking out of peoples heads so some people expectations can be quite low
As mentioned before it is incredibbly tiring as one person said to me at a wedding I did "you seemed to be every where I looked."
If you are contimplating earning a bit of money from photography then get some business cards printed as this will be a good oportunity to pass a few on.
Remain calm you know how to take good shots and enjoy the experience I know I have.
Good luck