Stick to the common language of the channel. Always try to moderate your language. Never, under any circumstances, hassle or abuse another user. Irc in general and if serious enough can lead to your Internet connection being terminated. This can cause the flood recipient to be disconnected as Irc servers cannot handle excessive amount of information.
Flooding can also lead to the penalties mentioned above.
It isn't necessary to greet each individual in a channel. A general greeting to everyone is the best way. Generally keep your behaviour pleasant and friendly. Remember, people will judge you by what you put on the screen, so take a second to think before you type.