Never give out identifying information - your home address, phone number, school - in a public messaging area like chat or user groups.
People at the other end of an email are not always what they seem. Someone claiming to be "she" and 14 could be "he" and 40. This may not matter but think carefully before giving personal information out, or developing a relationship.
If you come across messages that are deliberately provocative, obscene, racist, illegal, pornographic, threatening or that simply make you feel uncomfortable do not respond to them. In doing so you would be opening a dialogue with the person who posted the message.
If you receive messages that are harassing or otherwise disturbing talk about it with people you trust and forward the message/s to your service provider with a note about your concerns.
Just because it comes out of a computer doesn't mean it is true! Be skeptical of information of the net until you have identified its source. Seemingly credible stories can be invented and circulated for many reasons (for example political advantage).
Any offer that is "too good to be true" is probably neither 'too good' nor 'true'!
Put computers in social areas. Our own (and our friends') social rules and habits are a good check of our behaviours. In schools don't place monitors facing the wall, in homes get computers out of the bedroom. It will be worth the move for the discussion around the screen that will result.
Never, ever, ever arrange to meet as a result of an electronic contact unless parents and / or teachers are aware of what is happening. If you do meet, make sure the first meetings are in a public place with friends / parents / adults that you know present too.
Don't panic! There is simply so much that is good and useful and exciting on the net; working together on projects and tasks will always be less likely to cause problems than just browsing around endlessly. Talking about what we all discover and exploring each others discoveries will all help to build constructive use of time. The annotators pages in Schools OnLine are a great starting point...