I'm going to have to agree, somewhat, with mavrik.
If, that is, you do not feel comfortable shooting something of this magnitude. I say "magnitude" in the sense that this is someone's wedding. You can't reshoot photos you miss or flub up. (I speak from experience!)
If they cannot afford to hire someone with more experience, or they just want to give you the opportunity, then do everything in your power to PREPARE for the wedding day: research, research, research, take test shots of the two of them, the location of the ceremony, take close-up practice shots of the bride... Obviously, they must have seen something in you or your work to even suggest this...so, there is that.
There is much to be done that can help to prepare you, but once that day arrives, you just have to be ready to capture everything you can, as best you can.
One tip (from my wife) is don't be afraid to get up close to them. Don't interfere in the proceedings, of course, but don't let a shot escape you because you weren't certain. Take the chance, take the shot...they will thank you later.
That's about all I can say in a few paragraphs...but remember, there have been hundreds of books and manuals printed on this very subject. Research everything you can between now and then...or, again, if you don't feel comfortable, ask them (or research for them) to find other photographer options.
GOOD LUCK!
(We all have to start somewhere.)
(You can check our site below for some sample ideas, as well.)