what's the American tradition way?

Ladies, so One of my very good friends will be getting married in August, we are having lunch Tuesday and I know she has a little gift asking me to be one of her brides maid and I know that for sure. My question is do I just say yes in person or do I give her a gift few days later In a way of saying yes? Lol I just moved here couple years ago from a doffeeent country