Using

If you haven't used IRC before...
Simply put, IRC is a chat protocol, defined by official Internet standards, and capable of stealing many hours of your life. To use IRC, you must connect to a server, and then join a channel (equivalent to a chat room).
To connect to a server, open and select Connections New Server... or press F2. In the dialog box that is displayed, select a group from the leftmost drop-down box and then a server from the middle drop-down box. The port, shown on the right, will almost always be 6667. A new window will be displayed showing the messages sent by the server to the client. When these messages have finished scrolling past, you want to choose your channel. To display a list of available channels, type /list. To join a channel, type: /join #channelname. To leave a channel, just type /part.
That's the basics, for more detailed information, read on...

Basics

Servers
allows you to connect to an arbitrary number of servers. To connect to a server, select Connections New Server or press F2.The Connect to Server Dialog appears, and you can select a Group, then a Server, and also a Port to connect to.
Just click Connect when you're done, and a new window for the server is displayed. You can keep track of the servers you're connected to in the Server Control window.
You can get help on most IRC commands by typing /help commandname. Typing /help on its own gives a list of available commands.

Channels
You can connect to channels with the normal IRC command /join #channelname , by selecting Connections Join Channel... or by pressing F3.
Channels you are connected to are shown in the Server Control window under the server to which the they belong. To leave a channel, you can /part, select Channel Close or press W.

Channel Options
The Channel menu, available in any channel window, contains some useful options:
S Save to Logfile... 	 Saves the contents of the channel to a file.
Time Stamp 	 If selected, prepends each thing said in the channel with the time it was said, in the form [HH:MM:SS]
C T Ascii Table 	 Displays a table of Ascii characters, from which you can choose any characters you require.
C P Beep on Change 	 If selected, emits a beep every time something is said on a channel.

User Options
In a channel, the user list is displayed along the right hand side. Right-clicking a nickname in that list gives you a pop-up menu of options, most of which are self-explanatory:
Refresh Nicks 	 Sends a message to the server to request the list of users in the channel. This ensures that the list of nicknames is up-to-date.
Follow 	 Follows the user into whatever channels he goes to.
unFollow 	 Stops following the user.
Whois 	 Sends a CTCP whois query to the user, which provides information about the user, such as his hostname and what server he is connected to.
Ping 	 Sends a CTCP ping request to the user, to determine his echo time.
Version 	 Sends a CTCP version request to the user, to determine what software he is using.
Abuse 	 Abuses the user in one of various amusing ways.
The following commands are only available if you are an operator on the channel:
Kick 	 Removes the user off the channel.
Ban 	 Bans the user from joining the channel.
unBan 	 Removes the ban on the user joining the channel.
Op 	 Gives the user operator privileges.
DeOp 	 Removes operator privileges from the user.