The chat window is supposed to help you navigate through the channels that you've joined.
Right-click on the room's tab to quit that channel, but you cannot quit your corporation's and the terminal's channel.
In order to chat in any channel, you will need to click on the typing area at the bottom of the channel, and hit enter both before and after your message. This feature is intended to prevent any unnecessary typos, when you press robot controlling keys, or other hotkeys. You can disable the need to re-focus on the window each time you wish to enter text by going through the Game Options menu. (ESC)
Channel Operator Abilities
If you have the channel rights, channel options can be accessed by right-clicking in the text area.
Only operators have the right to set the topic of the channel - new channels don't have topics.
To control access to the channel from other Agents, just set its password (right click, set password option). Only operators are entitled to do this. Be aware that the channel can still be seen from the channel selection menu (as well as the number of users in the channel) but access to the channel will be restricted.
Channel Operator Controls
To reach these options, just right-click on an Agent in the chatroom and select the chat options section. You can promote the selected agent to the operator of the room (and later deop), or mute him/her.
The founder of the channel will automatically become the operator of the room. Other Agents can only be authorized by the original operator.
If you want to join another channel or create a new one, click on the Channels button in the upper right corner.