Hi Selva
if you know the things in Material Management, the flow and the work process of MM then you go for it.
working with purchase orders, vendors, purchase requisitions, stock inventory, etc
if you know this MM will be a good module for you.
and certification will be help you.
with my knowledge i would suggest you to go for SD or CRM as you have experience from customer service industry.
SD is basic functional module and will be easy to understand and work in while having experience in customer service industry.
better once you check the content of these modules over internet and decide the one which suits you more.
please refer below links if it helps you
http://help.sap.com/printdocu/core/print46c/en/data/pdf/caarcsd/caarcsd.pdf
http://www.guru99.com/overview-of-sap-crm.html
http://www.erpgreat.com/materials/what-is-sap-mm-module.htm
Regards
Ravindra Kulkarni