Science

B.Sc in Astrophysics is a 3 year Undergraduate course. And, to get admission in this course you need to pass in higher secondary school certificate exam from any recognized board and you must have physics and mathematics in your class 12th examination. You have to score minimum 50% marks in your class 12th examination. The eligibility criteria vary from colleges to colleges:
1 Tata Institute of Fundamental Research, Pune
2 Indian Institute of Astrophysics, Bangalore
3 Birla Institute of Technology & Sciences, Pilani
4 Punjab University
5 Indian Institute of Science, Bangalore
6 Raman Research Institute, Bangalore
7 Inter-University Centre for Astronomy and Astrophysics, Pune
8 National Centre for Radio Astronomy
9 Osmania University, Hyderabad
10 Andhra University
11 Institute of Fundamental Research, Tamil Nadu

Hi Vidhan or Rahul,
First, there is no shortcut here. Application process is time consuming and takes a year almost. Take the SAT or ACT and also the TOEFL to start applying to Bachelor's degree courses. To apply for MS programs you must complete your Bachelor's degree and take the GRE. You must probably apply for another student permit since it will expire couple of months after your Bachelor's degree course is over. So its better you apply during the final year and also check with the visa consulate there. But as of now focus on applying for your Bachelor's degree, you can find out information for your MS once you go there. Visit university websites now and check the admission requirements, deadlines etc and start applying.