Auditing seems to be a pretty straightforward and well documented feature. Google had quite a few hits, but the second link seemed to be pretty specific. Just be aware the the adump directory in your admin directory might get rather full. Google is your friend and big brother!!!

http://www.google.com/search?q=oracl...nt=iceweasel-a

http://www.oracle-base.com/articles/...ting_10gR2.php

http://download-uk.oracle.com/docs/c...6521/audit.htm