To participate in an IRC chat, you will want an IRC client. We recommend the following:
Windows: mIRC--download page
Mac: Ircle -- download page
Linux/Other flavors of -nix: BitchX -- download page
Linux users might also consider Gaim, a multi-protocol IM client that supports most IM services as well as IRC.
Basic Instructions: Start up the IRC client. Enter a user name and other required information. Find where it lets you choose a server, and select (or enter) "irc.freenode.net". Connect, then go to the channel where the chat is taking place. (If you can't figure out how to do that, typing "/join #(channelname)" should work.)