Once it's downloaded, install it. Once it's installed, run it. It should automatically open up options the first time you run it. All you need to do is fill out your name (doesn't have to be your real name if you don't want), email address (doesn't have to be your real email address if you don't want), and then your nickname and an alternative nickname (in case the nickname you choose is already taken). Then at the bottom, click OK.
Then, in the text bar, type /server irc.undernet.org and press enter.
when it's done connecting, type /join #originaltrilogy and press enter.
There you go, that's it. If everything works right, you should be able to chat with us.
Here are some other things:
To preform an action, type /me [insert action here]. It will show up as * [Your nickname] [your action]. For example, if you are EATING A PIE and your name is STORMTROOPER666, then type /me is eating a pie and it will show up as * STORMTROOPER666 is eating a pie.
To do colors, press ctrl+k and select what color you want to use. To do bold, type ctrl + b and for underlined type ctrl + u.
There are some other IRC clients you can use if you don't/cant use mIRC. Trillian connects to IRC, but I don't know any of the details on that so I can't really help you. If you're on a mac, then use Colloquy. A lot of people I know have said it's pretty good, and Jay uses it, so if you need help with it, ask him!