Hello aspirant, 

You can follow the following career pathway to become an astronomer:

You must excel in physics and mathematics as its quintessential in high school towards being a good astrophysicist.Graduate with either a BTech degree or a BSc degree in any domain that you're interested in from a reputed college. Doing it in either a science or mathematics field would prove to be extremely useful. Doing an internship from NCRA, IUCAA, ARIES, Vigyan Prasar etc would get you that much closer to fulfilling your dream.

 However, if you're an exceptional student, you can apply for the Dual Degree Program at IIST. IIST provides seats to students based on their JEE scores and also takes their grade 12 marks into consideration. IIST provides you with a BTech degree in a tenure of 3 years and then depending on your CGPA, the college assigns you the subject that you'd pursue your masters in. They only enroll 20 students a year of which only 5 of them get into Astronomy .MScPhysics is only taught in a few colleges in India like IISc, IIA, and IIST. Colleges like DU and Pune University offer a few courses in astronomy in their MSc Physics courses as options.Varied colleges like CAA, IIST, NCRA, TIFR, ISRO, ARIES, IISERs, ICSP provide a Ph.D. The criterion to do a Ph.D. is mostly a strong background in Physics.

Cheers!