ERROR: JAVA_HOME not found in your envir

i am trying to install webmail on linux it is a
tomcat based application.
the tomcat 4 server is running fine on the
server but when i try to install webmail i get
the following error
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WebMail 0.7.7 Build System
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Contributed by Nathan Chandler

ERROR: JAVA_HOME not found in your environment.

Please, set the JAVA_HOME variable in your
environment to match the
location of the Java VirtualMachine you want to
use.
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kindly please help me out in setting the
JAVA_HOME variable

Zahid Hakeem
Asst. Systems Administrator
Usman Institute of Technology