Internet Relay Chat is probably one of the oldest forms of "instant messaging", predating AIM and MSN and all those popular ones now. People come together in channels (or communicate privately by messages) and... just talk. Maybe send files every now and then.
Being an old protocol, there's tons of IRC servers out there to connect to.
You'll want to grab the appropriate client (program) first.
For Windows users, the most popular client is mIRC. You may also have heard of pIRCh, as well as various other not as popular clients.
For Mac users, the popular client is Ircle. Other contenders include Snak (the webslave's current choice).
Most Unix/Linux/*nix users should be able to access IRC by means of command-line clients like ircii as well as graphical clients (dependent on your GUI). Contact your system administrator for details. If you are the sysadmin and you don't know, GET TO LEARNING FAST. :p
Once you've got your client installed and set up, you'll probably want to know what to do with it. Refer to each client's website for usage instructions, and irchelp.org is a massive website detailing just about everything you'd need to know. I strongly recommend taking the time to memorize the basic commands (/me, /msg, /whois, etc.) (top)
EsperNet is the network #mobius is hosted on. It's on a bunch of servers mostly concentrated in North America and western Europe. You'll do well to go through their website and learn about their policies.
EsperNet uses a variant of the DALnet Services suite (ChanServ, NickServ, MemoServ, OperServ, etc...), which you'll get to know if you don't already do. Message each service for instructions with /msg servicenamehere help. (top)
EsperNet has just under ten nodes to connect to, but if you're just starting out, it's better to just connect to irc.esper.net on port 6667. Refer to your client's documentation on how to do this, or type: /server irc.esper.net 6667. The EsperNet web page has a listing of all the servers and their physical locations.
And that's it. You're in. Be a good netizen - read the MOTD, obey all policies, ask questions when you can't find any answers, use common sense, and you're good to go. (top)