To connect to Yahoo or Gtalk from Linux you need to install any multiprotocol Instant messaging client like: ‘Kopete’ or ‘Pidgin’
Kopete >> http://kopete.kde.org
Pidgin >> http://www.pidgin.im
Download them from the above links & install them.
To configure kopete for yahoo/MSN/ICQ/AIM etc:
http://www.freesoftwaremagazine.com/art
To configure Pidgin for Yahoo/MSN/ICQ/AIM etc:
Click Accounts >> Add/Edit. On the new Accounts window click Add.
Select the Protocol like Yahoo, Google Talk etc. & fill in other required details.
To configure Kopete for Gtalk:
http://www.google.com/support/talk/bin/
To configure Pidgin for Gtalk: http://www.google.com/support/talk/bin/