okay, there are two basic commands you are going to need to know:
first command= /server irc.mzima.net 6777
second command= /join #rantcentral
in your command line area on xchat, just type in the first command and it should connect you to the server. once you are connected, type in the second command and you will find yourself in the rant central chat room.
there are lots of other things to know, but let's focus on getting you connected first.
if for some reason, you cannot connect to that server, you can find more server to use at the link below. also, on that program you downloaded, there should be a sever window that lists efnet. you can select that and click connect too.
http://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=2&ved=0CGkQjBAwAQ&url=http%3A%2F%2Fwww.efnet.org%2F%3Fmodule%3Dservers&ei=rdPrT9i-IOLM2AWd4NTfAQ&usg=AFQjCNGHI4IfO6Cxdqf-YtFVqoPHgVgz7A
its usually best to pick one that is closest to you... but with broadband speeds... it's not really that big of an issue.