Hi
I'm not running Mandrake, but I'm sure the settings for SuSE and Redhat won't really differ from a Mandrake installation. First, I gather that you have installed either a copy of a jre or a sdk. You need the jre to allow you to run java programs, while the sdk allows you to both complie and run java programs. If you haven't, download one and install. Once you've done that, all you really need to do is add the installation directory's bin dir to your PATH variable. You just need to decide if you want to make it available system wide or only for specific users.
For specific users you can for example edit their .bashrc file if they use the bash shell or even just their .profile file.
For a system wide change you'll need to edit /etc/profile once again adding the installation directory's bin dir to the PATH variable for example:
PATH=usr/lib/jdk1.3.1/bin:$PATH
export PATH
(The example above assumes that you've installed the sdk in usr/lib/jdk1.3.1.)
You should now be able to run java programs.
Hope this helps.