Choose the type of computer you use...
I don't use a Mac, but our dear sister Nanners was inspired to write instructions for those of you who do. All the words of wisdom below are hers exclusively...
The first thing you must do is download the chat program IRCle. The web page is: http://www.ircle.houseit.com/download.html. Other good download sites to try are http://www.xs4all.nl/~ircle/ and http://www.tucows.com
When you're done downloading the file, if your computer tries to tell you that it's an "unknown file type" and prompts you to make a decision, pick the "save file" option. Then drag and drop the file onto stuffit expander and it'll open it. (I had to download it three times before I figured it out, but my computer is a few cans short of a six pack).
Double click on the IRCle logo in the folder - but I'm sure you KNEW that part, right? You'll have several windows open, and you'll need to tile them a little better to fit your computer screen so that you can see what you need to see.
There should be a little window named "connections" - highlight the first line in that window and then click the "edit" button in that window. That should open up a new window that has three tabs in it - you want the "identity" tab. The first line says "server:" and than has the name of a server that you probably don't need and then a button that says "select" - hit that button. The server connection window will open and you want to click "add" which will give you a box to enter the server you want. Type in us.chatnet.org for the server name, any number between 6661 and 6669 for the port (I wouldn't use 6667 because it tends to be slower), and then pick ChatNet for the network. Then click okay again, and you'll be back to the "identity" part. Pick your nickname and put it in the nickname block, pick a user name for the user name block, and then enter your real name if you want to where it asks for your real name.
To change your preferences, go under "file" in the menu bar and pick preferences - there are a lot of them, so you can just sort of play around with them. The most important one, IMHO, is to go under "text" and uncheck the box that says to show the MOTD - message of the day, which can get really long and annoying depending on which server you end up getting routed through and how much of a hurry you're in to connect.
If you're using a newer version of IRCle, it's going to eventually get annoyed with you if you don't pay the shareware fee, and it'll ultimately start kicking you off the server every 30 minutes. You have two choices - you can either pay the $15, or you can go in your system folder under preferences and throw away your IRCle preferences file, which I don't recommend because you lose everything you've fixed how you like it and have to basically start over again. If you're planning on chatting a lot, it's well worth the $15.
If you're trying to have a private conversation with someone, and you're typing /msg nick to talk to them, their messages are going to come up in your main chat window and you'll hear a little telephone sound - to reply to that person, you'll have to type /msg nick every time you want it to go to them only. If you want to open a NEW window with that person, you have to type /query nick and then your message instead - same concept, but that will open a new window and you won't have both conversations in the same place. Once you're in that window, you can just type your message like you usually would and don't have to worry about it going to the wrong people.
Hmmm - I didn't mean for that to get so long... just e-mail me if you're confused - I made it so much more complicated than it really is, I think!
For additional help with setting up IRCle, try http://www.irchelp.org/irchelp/mac/ircle_setup.html