There are a couple of good threads going on this forum with lots of good advice:
Pentax Weddings - who's doin'em?
One of the members here pointed me to this a long while back, but it has some great pointers and is a good place to start:
Wedding Photography - 21 Tips for for Amateur Wedding Photographers
My advice would be, have back-ups for at least the body (beg, borrow, rent, steal, or buy and sell it later on), and preferably a couple of good flashes as well. Bring more memory cards than you think you will need, and bring more batteries than you think you will need. Keep it simple on the event day, you don't want to be messing around with radio triggers on your first wedding....
They are very challenging but a lot of fun and quite rewarding if it goes well. Do the legwork ahead of time and nobody will know it's your first time....