The world of Physics is a fascinating one and today the sciences are merging with each other at higher levels and at that level there is no distinction in Physics, Chemistry, Biology and Mathematics. A Physicist is talking of Quantum Biology to decipher the origin of life. He is using Chemical reactions and Mathematical models to unravel the mysteries of nature.
So if you have a desire to become a theoretical Physicist then you have to work hard in Physics and Mathematics but should take along with you the concepts of all other branches of science like Chemistry and even Biology also.
The best course will be to go for your graduation in some of these subjects as available in the curriculum of the college or university where you plan to study and then go for Post Graduation in Physics with specialisation in the subject of your choice may be Astrophysics or Astronomy. Thereafter you must pursue research in the field of your interest and then can try to join some leading scientific institution like TIFR, IISc, BARC etc for further growth of your ambitions in the research area.
In our country all the IITs are a good place to pursue a career in Physics only thing is you will have to qualify in their entrance exam which is held on all India basis. Then there are other Institutes like - Birla Institute of Technology and Sciences, Pilani, Institute of Technology, Banaras Hindu University, Varanasi, Vellore Institute of Technology, Vellore, Indian Institute of Science, Bangalore, Indian Institute of Science Education and Research, Pune, Mohali and Kolkata where also one can try to get admission through joint entrance exam.
Apart from above, there are some other good colleges where one can try to get admission and pursue one's dream career. They are - Presidency College, Kolkata, St Stephens College, Delhi, Christ College, Bangalore, St Xavier's College, Kolkata, Fergusson College, Pune, St Xavier's College, Mumbai, Loyola College, Chennai, Madras Christian College, Chennai, Presidency College, Chennai, Madras Christian College, Chennai.
Knowledge is power.