If my sister is being solicited online, how can I tell the police?

My young sister, despite numerous warnings to be more careful, gives her email address out on chat sites and pedophiles add her and try to chat with her about . This greatly disturbs me and I would like to try to tell the proper authorities when this happens to her and have the guys caught and arrested, or at least scare them/get them in trouble. How can I go about doing this?