IRC is like most common chat-programs based with a server, which users (like you) connect to with a client-program (mIRC, HydraIRC etc.), which you can download from one of the links below.
I could go on and on with technical-babel and features how it works. But there are a lot better pages out there. I would suggest to begin with each IRC-client's homepage, with how you connect (to a server) and join a channel. When you managed to join our channel, I will help you out with any questions regarding IRC if I can answer them if you have any by then.
Be sure to check by sometime. :)