Log off and tell a trusted adult. Never agree to meet with someone you've talked with on the Internet without first asking permission from your parents. If a friend on the Internet wants to meet, make sure you get your parents' permission and have one of your parents go with you. Always trust your instincts, if you do not feel comfortable giving information, then do not. Never send a person your picture without checking with your parents.
Many sites use passwords.
Do not forget to ask your parents before you enter any personal information. Adults can pretend to be kids. Plenty, according to the experts. Many of the same rules apply. Monitor your child's use of on-line services, just as you would any of your child's entertainment activities.