On Windows, the best client is probably Pidgin. On Linux, Kopete is actually slightly better than Pidgin. iChat (included with OS X) and Adium are both popular chat clients with Jabber / XMPP support for Mac OS X.

At first you need to choose one of the jabber servers. Try to find some by google, or wiki pages

Than you need to download some jabber client. Try this page for example:

http://www.jabber.org/web/Clients

Once you have chosen server and client application, you can get your account. Most of servers doesn't need to register on-line by some form, just run client application and it will ask you for the login, password and server which you want to be connected to.

At first you need to choose one of the jabber servers. Try to find some by google, or wiki pages :)

Than you need to download some jabber client. Try this page for example:

http://www.jabber.org/web/Clients

Once you have chosen server and client application, you can get your account. Most of servers doesn't need to register on-line by some form, just run client application and it will ask you for the login, password and server which you want to be connected to.

No. To use Yahoo, you'll have to get either Yahoo's official messenger, or a third-party client like Adium (http://adiumx.com) or PSI (http://psi-im.org). If you have a jabber account with a Yahoo transport you can setup PSI or other jabber client to register with that account and then add your jabber account to iChat. See http://timmargh.net/archive/2005/05/use_msn_yahoo_o.php for details.

You don't. You can use a general-purpose Jabber client like Psi or Pigdin to communicate using GTalk.