All personal information are confidental. Don´t give out any e-mail, contact information, phone number, bank account number, etc. Remember that also your photo is personal information!
Don´t believe everything you read or see on-line.
If something on the net feels you uncomfortable, tell to adult.
Don´t meet with people you know just from the internet.
Your password is your private matter, don´t tell it to your friends.
Don´t reply to vulgar or harsh e-mails. And don´t reply to anybody you don´t know.
Educate yourself. Learn new things, confirm your sources.
Don´t forget: always be careful!