Hi Kamlesh.
See these links.
A functional person need not go that deep or fundamental to make use of a User Exit.
When you have a task i hand and identified a exit, then writing a simple code does not require learning theory in this area. Initially an ABAPer nearby can help you. Later you will be able to pick-up.
However go through these links:
http://www.saptechies.org/user-exits_1/
http://wiki.scn.sap.com/wiki/display/ERPLO/PM-CS+User+Exits
Jogeswara Rao k