The Freenode
To use IRC you require an IRC client. Many options exist, and popular choices include Xchat, irssi, Chatzilla and Colloquy. Most general-purpose chat clients like Pidgin and Kopete can also be configured for IRC. Usually all you need to do is choose a nickname or change the default, connect to irc.freenode.net and join the #octave channel.
Freenode provides a web client if you prefer to not use a program external to your web browser. For more information about Freenode itself, including how to register a permanent identity within the Freenode servers using services like Nickserv, consult Freenode's own documentation.