Hi mksingh,
That is quite a broad question/topic, but I would recommend you start here:
The actual configuration and logs you monitor will depend heavily on the flavor of the nix system (RHEL, HPUX, AIX), but as RHEL is mostly what you might see...check out the docs on the Linux audit system.
A couple good resources here as well:
https://www.function1.com/2015/07/splunking-the-linux-audit-system
https://splunkbase.splunk.com/app/2642/
http://security.blogoverflow.com/2013/01/a-brief-introduction-to-auditd/
Your best bet would be to consult a security and compliance SME in your field to ensure you are capturing what matters to securing the systems in use.