Which OS are you running on your iMac?
One of the "built-in" applicaitons of OSX (any version) is iChat. It contains 3 modes of communication: text, voice, and video. To use video requires a CPU running well beyond your iMac (plus a camera), but you can surely use text, IF you have the iChat application.
Open it, and a window will appear on your desktop (the "Buddy List"). Register yourself as a wannabe iChat-ter by using either a dot/Mac account, if you have one, or an AIM account (free) . . . found at . . .
AIM Account Registration here, for freeThen, get your friend's ScreenName, insert it on your Buddy List
via Add a Buddy at . . .
[attachment=783:AddABudd...nu_item_.jpg]
When your friend is on-line, and on IM, his/her name will show in your BuddyList window. Click on it, and start chatting when the friend "answers".
As Al said, "No stupid questions on TeeEss", just folks with puzzles the rest of us enjoy solving, if we can.
PS: If you don't have iChat available, I
think you may be able to do the whole thing via AIM. AOL used to "guard" it for its subscribers only, but a couple of bouts of litigation changed that. iChat is the "method of choice", however, if you have it.