There are many many people in cyberspace who are generous, helpful and kind. But it only takes one rotton apple to spoil the experience. Do not freely reveal your email address or location. Do not expect people to be what they say they are. Be careful what programs you download and where you get them.
Computer viruses are very real.
Guard your tongue and your temper. It is very easy to become angry and flame other people. This leads to no good end. No argument was won by beating someone over the head with stick or tongue. Likewise, don't let the flames of others singe you.