From iChat 3 to iChat 6 and then Messages the basics have remained the same.
I have a G4 tower and it came with OX 10.2 and soon after that iChat was released in it's AV form (iChat2)
The Tower can run everything up to Leopard (OS X 10.5.8) and therefore iChat 4.0.9.
You haven't actually said which version you are running. (OS or iChat)
In the Applications folder you have highlight the app and then do a Get Info (Apple key + i key together ( or ⌘ + i) )
This will tell you the version of iChat you have.
If it is iChat 3 or 4 it will be able to join the AIM and the Jabber services.
As Starting point you can register for AIM names here.
You have to enter a real First and Last name and then Test a Screen Name and add and confirm a Password.
You will need to use a recovery email and list answers to Security questions for Password Recovery and the like.
Apple Isssued IDs can also be be valid AIM names but due to recent changes in Lion (iChat 6) and Messages there can be issues with using them in earlier versions.
If you have an @mac.com ID this will work in any version of iChat and Messages providing your password is 16 characters or Less (An AIM server limit)
In IChat 6 and Messages the iCloud Issued IDs are held on a different server and as a Check the application logs into AIM and to Apple. The login to Apple then lets AIM "read" the password.
These are the IDs ending in @me.com and @icloud.com (these will not work in iChat 5 or earlier)
SInce iChat 3 you can also use Jabber IDs.
GoogleMail IDs that have had the Talk option enabled (In Products) can be used as a Jabber ID (referred to as Google talk)
Facebook IDs that have had "Chat" enabled are also valid IDs
There are many other Jabber servers around the world.
These are an interlinked network of Private and Public servers.
Since iChat 4 the Adding of Googletalk IDs has had it's own selection (it is due to Google not following a general rule of thumb over server names)
Once you have an ID or screen name you want to set up then you need to do the following.
Go to the iChat Menu > Preferences > Accounts
At the bottom of the list on the left click the + icon
In the new pane click the top item to show the drop down options.
Broadly speacking they are in two groups ("AIM" and "Jabber")
The AIM subset will likely offer you @mac.com, MobileMe and AIM
The Jabber Subset should offer Jabber and possibly Google depending on Version.
The MobileMe option will not work for @me.com addresses as these have moved server for Verification of the password.
As I said above the @me.com name and the @icloud.com names will not work in iChat 3, 4, or 5)
The @mac.com option should still work but if it does not (and your password is 16 characters or less) use the AIM option but enter the ID in full.
For Jabber IDs use the Jabber option.
In iChat 3 you need to use this for the GoogleTalk option.
Once you have clicked the Done button and the Buddy list has appeared for a Jabber list go to the server settings tab
Some Jabber servers use SSL (It will automaticaly change to port 5223)
Others do not (port 5222)
In iChat 3 change the Google Server name to talk.google.com
Get an Screen Name (AIM) of Jabber ID
Open the app and go to the Accounst part of the Preferences
Click the + icon
In the new pane select the most suitable option in the top item drop down.
Change server settings on Jabber accounts if required.
iMac 2.5Ghz 5i 2011 (Mountain Lion 10.8.4)
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