What subjects are involved in robotics. If I want to build robots then what necessary things I need to learn consecutively as a beginner.
To give you an answer that is similarly broad to your question: Mechanical engineering, electrical engineering, control theory or computer science are a good start.
To give a more in-depth answer, you would have to tell us a little bit more. For starters, are you interested in robotics as a hobby or study, and what is your background? Is there any specific field of robotics that interests you?