>>2
You can use a Web client like Mibbit (
http://www.mibbit.com/) to get on a channel without installing any software. You can visit
http://www.irchelp.org/ for more info on how to use commands and such. It's not really difficult to learn, and it's quite easy once you get the hang of it.
Once you get tired of using something like Mibbit, and are looking for an actual client for your platform, here's a few.
Windows:
http://www.mirc.com/ http://silverex.info/news/
Unix/Linux:
http://www.irssi.org/ this is what I use myself, on my provider's shell account.
Mac OS X:
http://colloquy.info/ http://www.ircle.com/ http://www.sysctl.co.uk/#macirssi also, since OS X is based on BSD Unix, some Unix/Linux clients can be made to work on it as well.