You do need some form of Screen Name or ID.
Screen Names tend to be the format for describing those names that work over the AIM service
(AIM's Own Names, Apple IDs issued by Apple such as @mac.com and iCloud issued @me.com and @icloud.com)
IDs tend to be the way Jabber or in iChat 6 Yahoo IDs are listed.
There are many Jabber Servers that you could get an ID from. Google run a Jabber server for Gmail IDs and these tend to be referred to as GoogleTalk IDs (You have to enable "Talk" on your Google Settings)
Facebook (enable "Chat") also run a Jabber server based on your Facebook ID.
Yahoo, like AIM run a central server holding all the names that are in use.
Jabber (or XMPP as it is also called) is decentralised with servers allowing names from different servers to appear in people's Buddy lists.
The Get a Screen Name page is now a bit out of date. Apple stopped MobileMe and AIM moved their Registration Page (to Here).
When you have a Screen name of Jabber ID ready you can start up iChat
If you have not run it before it has some Start up Screens (see the next menu item in that How to Link)
iChat 4 as several choices in the AIM section (offering @me.com and @mac.com as well as AIM options)
Be careful how you enter the names as some have suffixes added for you.
You get a Jabber section to add an Jabber or Google ID.
You can Enable the Bonjour (Mac to Mac on your LAN) option.
You should now be ready to Add Buddies to the list(s) you have created.
Extra accounts can be added later in iChat Menu > Preferences > Accounts and clicking the + button lower left below the list.
In this pane the top items is a drop down offering AIM based option above the line and Jabber ones below.
At iChat 4 Google has it's own option to cope with it's Server name issue (they don't follow the Jabber standard).
You can use this if you clicked through the Start Up Screens without setting anything up.
Please, if posting Logs, do not post any Log info after the line "Binary Images for iChat"
iMac 2.5Ghz 5i 2011 (Mountain Lion 10.8.2)
G4/1GhzDual MDD (Leopard 10.5.8)
MacBookPro 2Gb (Snow Leopard 10.6.8)
Mac OS X (10.6.8),
Couple of iPhones and an iPad
"Limit the Logs to the Bits above Binary Images." No, Seriously