Well, you would have to put the path to your jre at the bottom of your /home/username/.bash_profile or your /etc/profile. Something like this:
This will not work straightaway; log out if you use bash_profile or simply use this command (must be root) in the other case: source /etc/profile.
You can also set it up just for the duration of a session by starting up a terminal and entering:
e.g. export PATH=$PATH:/opt/jre1.6.0_04/bin