Most of the admissions to post graduate studies are on the basis of entrance examinations. After completion of B.Sc Physics, you can go for specialisation in astrophysics.
Mahatma Gandhi University in Kottayam, Kerala, offers M.Sc with Astrophysics specialisation.
Osmania University,Hyderabad and Pondicherry University offer a similar courses.
The admissions are through entrance examinations. You can visit the respective websites for more information. For entrance exam preparation, go through the previous question papers if available. Also there are a lot of preparatory study materials and books available. Go through some reviews and select the ones you find good.