Like her best friend Laurie, Amy is pretty, smart, and popular. Unlike Laurie, Amy doesn't always show how smart she is. The novel suggests that Amy hides her excellent math skills because she's afraid they will intimidate Brian Ammon, the football player she likes.
Amy lacks Laurie's self-confidence and is actually really jealous of her friend. Jealousy and insecurity seem to open Amy to The Wave. She sees The Wave as a way to cut through the competitive atmosphere at school and free her from what she sees as pressure to compete with Laurie.
Without knowing Amy better, it's hard to say why a person with so much going for her isn't more confident. What do you think?