Just as we tell our children to be wary of strangers they meet, we need to tell them to be wary of strangers on the Internet.
Most people behave reasonably and decently online, but some are rude, mean, or even criminal.
Never share their password, even with friends. Never respond to messages that make them feel confused or uncomfortable. They should ignore the sender, end the communication, and tell you or another trusted adult right away. Never use bad language or send mean messages online. Also, make sure your children know that people they meet online are not always who they say they are and that online information is not necessarily private.