I actually use Pidgin now so that I can keep all of my IM accounts open at once with only one managing window open. I really like it. You can even add Google chat to it now! So, if you've got a gmail account it's handy, too.
As for AIM (AOL Instant Messenger), you can go here to get a screen name without downloading the software (then you can use Pidgin or another managing account) or you can go here to download the software and get a screen name at the same time. If you don't want to have the hassle of having all of the software on your computer, you can go here and launch AIM Express every time you want to chat. I personally don't like having all of the software on my computer--AOL can be invasive--so before I had Pidgin I used Express.
When you get your account set up and your buddy list up, you add your own name to the list (from the drop-down menu on the top) then either right-click on your name and invite yourself to a chatroom, or you add the whole chatroom to your buddy list. I'm not sure if AIM lets you put whole chatrooms on your list, though, now that I think about it...