How you set environment variables depends on which shell you are using.
Typically you would use:

for sh:
$ set VAR="some text"
$ export VAR

for ksh, bash:
$ export VAR="some text"

for csh:
$ setenv VAR "some text"

Suns java compiler does not require any environment variables in order to
compile java code. Which compiler/tool are you using to compile your code
and what exact error message is it producing?

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i thnk its the BASH shell .. this is the default Linux RH9 shell that
i have . i did tried to make some changes in the fiels and tried to
point to the libraries in the directory but still when i compiled a
few applications , they say that the JAVA libraries could not be found
.. and can i also know  the command to find diff app ... like i wanna
know where QT is installed ... so what to type on the shell to find it
?