"IRC" means Internet Relay Chat. You download a piece of software, connect to a server, and ta-da, you can come talk to us in real-time. Wheee!
If you're using windows, you have several options. You can try mIRC (download at http://www.mirc.com/) or you can use the windows client for xchat (the superlative IRC client by far) by downloading from the software repository
If you've got a Mac without MacOS X, go to http://www.ircle.com (which sucks, but there's not much available).
Once you have an IRC Client (mIRC, IRCle, xchat,) installed, tell it to connect to server castle.nbtsc.org, and join channel #nbtsc ("/join #nbtsc"). If you get confused with the connection set up, just get into the main window and type "/server castle.nbtsc.org" and hit return.
If you just can't get that to work, or a parent won't let you download any programs, you can try web-IRC. It kinda sucks, but it'll get you connected.