You need to be aware of which sites people can gain access to your profile and without your permission. Some examples are: Facebook, MySpace.
Beware of sites where people can change your password just by knowing your user name. An itunes account is a good example of where this is the case. If your not sure, test it out for your self by clicking on "forgotten your password?" and seeing whether the new pass word is accepted.
If you become a victim of cyberbullying, make sure you contact people responsible for the site.
If you join a forum, make should you read through the rules and regulations for members. This can be especially important if the forum is a special interest forum where you may get asked to leave if you post "off topic" mails.
Use the Internet respectfully: don't post embarrassing pictures of your friends onto sites without their permission!