Nice Summary of setting up audit options
The article is a fast ride through lots of options; turn on audit, sys audit, audit options, some forensics ideas, querying the audit trail, using statspack, database vault, audit vault, FGA, checksumming, flashback, log miner, system triggers, AWR, Materialised view logs, trace,DML triggers, even DBMS_SYSTEM.KSDWRT, alert logs and even the scheduler logs. This is a great article that contains a lot of valuable data on what you can do for free in terms of auditing and monitoring your database. Well worth a read.