Dear Shankar,

It is all about your interest. Find yourself a subject that you like most and matches with your basic skills such as: reasoning power; mathematical skills; creativity; fascination..etc. If you lack skills, develop them during your Masters degree. Find the best institutes that allow you to conduct research in your field and get in touch with them to know about the basic requirements and selection procedure.

You may write directly to the professor in the institute. You may directly join for integrated PhD programs. You can also participate in internship programs provided by many institutes which gives you a good exposure to scientific research. Once again, selecting a subject for your PhD must be based on your interest and skills.