Hello...
you need tofocus on physics, mathsand, to an extent, chemistry as well.You'll be required to do BSc and MSc in physics.Masters can be done with specialization in any physical science subject like Material Science, Applied Physics, Electronics etc. However, you should be reasonably clear with your understanding of Classical Mechanics, Electrodynamics, Quantum Mechanics, Statistical Mechanics and Mathematical Physics as these are the fundamental subjects in Physics.

Few universities also offer MSc in astronomy. You can also do that after your BSc in physics. During the summer vacations after your BSc or during the MSc you can apply to national research institutes in India, where they do research in astronomy, for summer internship. This will give you flavour of the environment in research institutes as well as help build up your CV. Please make sure you grades are good, as that helps in getting these intern-ships, though they are not the only criteria.Whatever you study during BSc and MSc courses is your basic training for being an astronomer in future.

During and after completing your masters, appear for national level entrance exams like GATE, NET, JEST etc. Some institutes also have their own entrance exams. You need to clear at least one of those with a good percentile to be called for interview. Clear the interview and you'll be offered research scholarship to doPhD on a particular topic in astronomyunder the guidance of a scientist.A PhD can take anywhere from 3-7 years in India. However, most generally take 4-6 years.After that, you can go for for post-doctoral research or you can join a research institute or university as a faculty where you can pursue further research. One can even go to international institutes as well for doing PhD.
Hope it helps you.
Good luck .