If you have been invited to a wedding by a girl who's a bridesmaid, that sounds like a rather nice occasion and I hope you both have a good time. But neither of you owe the other one sex, and certainly agreeing to be the date for someone in the bridal party doesn't line you up to be intimate with her if that's not what you want to do.
You probably won't have to tell her anything in particular. She ought to be able to pick up on your body language and the way you relate to her - as a friend sharing a happy occasion rather than as a date with whom you are exploring possibilities for intimacy. But if things feel like they might later get out of control you could find some gracious way of indicating that you value your friendship but aren't interested in a romantic relationship with her. And watch, of course, how much you drink!
On losing your virginity, whenever that comes I hope it's with someone kind and understanding, someone to whom you can just honestly say this is your first time and then relax and enjoy the experience together.
Young men tend to think of this moment as a validation of their manhood and something of a "conquest", therefore the conquest should be as attractive as possible. Attractive sounds good, sure, but looking back on my first time with a girl I would have to say that what made it special wasn't her looks but the fact that she and I were good friends, both virgins, and both worried and anxious but wanting it to be special for each other as well as for ourselves.
Nobody living can ever stop me
As I go walking my freedom highway.
Nobody living can make me turn back:
This land was made for you and me. (Woody Guthrie)