There are a number of ways you can get onto the chat server.
Possibly the simplest way is to click the Chat link in the navigation bar. This will open the java irc client and, assuming you have java installed will connect you to the server. From there you just need to follow the instructions to enter a room.
You can also get your own IRC client software, one popular program being mIRC available as shareware from www.mirc.com
To connect to the irc server using mIRC, run the program and then from the status window type:
(Ports 6667,6668,6669 and 7000 are available).
This will connect you to the server, from where you can type /join #GUA to join the main room, or /list to see the other rooms open.