Wow! Shooting a wedding for the first time...brings back memories. (I do 3 or 4 weddings a year.) Ed hit it on the button. Unless you have spent some time with another photographer shooting a wedding or attending a number of weddings to study how the photographers work, reconsider. But if you do it, here are a few things to consider.
1. Have a back-up for everything -- camera, flash, batteries, film (lots), cables, did I mention camera & flash. Look into a high power battery pack.
2. Visit the site ahead. You must know the routine so you can plan where to be at the appropriate time. Anticipate. I try to attend the rehersals if possible. Know where you are going. Know the route from church to reception site.
3. Work with the bride (the groom is along for the ride) to make sure you cover everything. Make sure she takes care of anyone that gets in your way, but don't discourage other photographers from shooting your set-ups, 'cause if something should ever happen, their film might save you.
4. Shooting formals, it helps to have an assistant help arrange everyone. It speeds things up and keeps your subjects from wantering away.
5. Use a processor that you trust. If you show up with missing film, the family will be all over you like ugly on a monkey.
6. Be professional.
7. Finally, have fun and put the couple at ease. Just because you can NEVER go back and repeat the event doesn't mean it can't be a great day.