You are a Mac user so you already have iChat instant messaging software. To activate it you either need a profile with AOL or with .Mac but the .Mac profile costs money so we are going to assume you are going the AOL route.
Step 1 is to go to http://dashboard.aim.com/aim
Step 2 is to click on the Create a Profile link
AIM page with Create a Profile link

AIM profile is associated with AIM software

Click on the Try AIM 6.5 today link (even though you don’t need that software and are going to use iChat).
All you want is the Get a Screen Name Free

Click on Get a Screen Name Free
Create your Account with a screen name

Just click Continue
Picking a screen name

You might have to experiment here to find a name still available that is meaningful and easy to remember since many names are already taken. Keep trying until you find one you like and remember the password.
Security question choices

Pick the one you can remember and isn’t known by everybody else.
This security check is to stop programs creating accounts since it takes a person to decipher the image

If you have trouble reading the image click the link below to get another choice.
After you submit the request you will have a screen name and password
Start iChat and set it up with you new screen name and password so it will login to your new AIM account.
After all this which hopefully helped you get ichat setup with an AIM account. There is another alternative – the lazyweb.
It turns out there is a great article on this topic for just the price of a link:
Just go to http://www.mvldesign.com/video_conference_tutorial.html