First of all, to begin chatting, click on "log in" in the upper right corner of the box.
The rest is rather simple.
Codes:
/me | this code enters your username in it's place, it's to be used when you want to perform actions. For example "[username] kicks you in the head
/img | this code is used before a link, it does the same thing as the [ img ] BBC code.
/exit | disconnects you from the chatbox. If you want to put the reason you are leaving, that will come through as well. For example: /exit I need to go to the bathroom
/abs | this is pretty much the same as above, except it doesn't disconnect you, just tells people that you're gone. You can add a reason here as well.