"I Saw You That Night" by R. L. Stine

Sand. Surf. Sun. Roxie lives for the summer. But this summer things aren't going the way Roxie planned.

It all started when she broke into Lee Blume's house just to win a stupid bet. Because while she was in Lee's house, Roxie saw something she shouldn't have seen. Heard something she shouldn't have heard. And someone's looking for Roxie. Someone who wants her...dead.