Choose iChat > Preferences, then click the Accounts icon. If you're using an AIM account, enter your screen name and password. Your password is added to your keychain so you don't have to type it every time you log in to AIM.
For Mac.com accounts, enter your full email address (for example, tclark3@mac.com) and password. Your .Mac password cannot be longer than 20 characters.
For AOL Instant Messenger accounts, enter your AIM screen name (for example, tclark3001) and password.
For Jabber accounts, enter your full Jabber ID (including the name of the server, for example tclark@jabber.org) and password.
You can use iChat to communicate with people who use Mac.com, AIM screen names, or Jabber IDs. If your friends are using older versions of the AIM software, they might need to get a newer version of the AIM software in order to send messages to your Mac.com screen name.