This is pretty much simple for most people, however i tend to wipe the howto from my memory each time i reinstall pidgin.
so heres the basic setup (this does not include signing up your gtalk account)
Procotol: XMPP/Gtalk
Username: Dennis
Domain: annoying.dk
Resource: Empty
Password: mypassword (god,love,sex…)
Local alias (nickname)
Under Advance, keep it simple.
No checkmarks required in the first 3 boxes.
Connect port: 5222
Connect server: talk.google.com