First thing you need to do is get yourself an external flash...I use the SB 800, can't comment on the 600 or 900. Once you mount the flash on the hot shoe, you do not want to shoot with the flash pointing directly at your subjects unless you have a diffuser on your flash (does NOT apply when using the flash for fill light ONLY, such as outdoors in daylight...then bare flash is fine). Before resorting to the diffuser, try bouncing the flash off the ceiling or a wall. You are going to need to experiment with the effects of bouncing BEFORE the wedding...i mean, like now...you need some time with this and you have very very little time to work with. Bouncing flash can be great, or you can end up with half a lit face and a really really p.o'd bride.
All of the above said: A friend of mine shot a wedding for a friend (NOT paid) with a D5000 and the onboard flash and it looked pretty darn good...it is a good thing you have a Nikon DSLR. If you are getting paid for this though, practice with the external flash tonight. You do not want to "chance it" with the onboard unit.
Oh, fast lens. Do you have a fast lens so you can potentially use natural light (if there are windows and the wedding is during the day)? Fast lens...faster than 2.8. Preferably faster than f/2. If you can go without a flash you are in much better shape. Natural light is beautiful. Flash can be beautiful...can be tragic.
If you aren't getting paid for this wedding and didn't sell your skills to the bride to get a portfolio (in other words, if your friend saw you have a camera and said, "hey, can you shoot my wedding?"), then use your onboard flash and have fun. Chances are the photos will come out fine...they will still be far superior to a P&S.