Hi Mahesh
If you are new to SAP I recommend going through authorised training and using SAP's material. You can look at SAP Press Books for the book Authorisations made Easy. It's a good starting
If training cost is an issue, have you looked at SAP Learning Hub subscription? It will give you all material. As you are in India, if you want to go for Certification you would need to contact SAP India as they do appear to have country-specific pre-requisites. This may not be an issue if you are trying to self learn. As you said you have a job already, your foot is already in the door. Self-learning could be an option (if you were beginner/fresher with zero experience a formal training might help your resume better).
Regards
Colleen