please help !

I m new to linux . how can i write a policy that specifies that user FOO can only read file alpha and user BAR can only write file alpha and user FOOBAR can only execute file alpha .any other operation form any other user must be disallowed . i dunno how ? how can i customize permissions for different users ?

You can’t disallow access for root - the root account can access everything (unless the file is encrypted, and root doesn’t have the encryption key, but this wouldn’t really apply unless there were multiple people involved in using & administering the computer)

Anyway, chmod is what you want, there’s good info on wiki - chmod - Wikipedia
You don’t say what distro you are running, some have quite nice GUI tools for this (doesn’t hurt to know the terminal way though :))