some Linux commands

"HL" <[email protected]> Sat, 4 Oct 2003 21:44:11 -0700
Newsgroups gmane.linux.redhat.release.enigma
Message-ID <00a901c38afb$52d91a50$6501a8c0@oemcomputer>
Not familiar with Linux commands.

What kind of commands to use in order  to find out who, when and what they
did under Linux server ?
How to find out passwords issued for some accounts by previous system
administrator ?

Thanks