well, you can start by going to
www.mirc.com and dowloading the mirc client
also, that main mirc page has lots of info on what IRC is and what not
basically, if you know certain rooms to go into and certain servers to connect to, you can find lots of people with similar interests
the only major file sharing usually happens between people in forums that trade warez, movies, etc.
for starters, i can suggest connecting to any undernet server. once connected (you should be able to figure out how to connect)
you type "join #channel name" where channel name is where you want to connect on that server
i'd suggest joining #neochat
it's on undernet, and lots of AT'ers hang out there
you can create your own room as well by typing #leeland or whatever and telling people to join
it's kinda like old-school AIM