After your 12th you have to go for the course of BSc in Physics or Integrated MSc in Physics. Now this courses are offered by IIT's, IISc as well, if you wish to go for the JEE ADVANCED examination. Else, you can take admission in any of the BSc in Physics courses in state university. After the completion of BSc Physics, you can appear for the entrances such as TIFR GS, JEST etc., in order to go for the top research institutes in India such as IUCAA, IISc, etc. So after BSc you can go for the 2 years MSc program and then pursue your PhD or you can go for the Integrated MSc PhD program and you can conduct your research in Astrophysics. After your PhD, you can call yourself an Astrophysicist.

For more information regarding the Astrophysics as a career, consider the link provided below:

https://www.careers360.com/courses/astrophysics-course

I hope this helps. Wish you a great eve!