Hello Ninad,
There are already many discussion threads on this subject, but basically, if you want to learn SAP Payroll "from scratch" you'll have to learn the basics about Payroll in general, and then you'll have to learn some basics on the Legal Payroll Requirements in your Country.
Please consult the wikis on this site
ERP Human Capital Management - ERP Human Capital Management - SCN Wiki
and the documentation on help.sap.com