As you want to become an astronaut after completing 12th class so sorry to say you but you are not eligible for that.

Firstly you will have to do B. Tech in Astrophysics and then you will have to do M. Tech and then you will have to do PHD.

And than you can be eligible for applying entrance exam of Astronaut.

You can do PHD in following department:

PhD (Astrophysics)
PhD (Geophysics)
PhD (Metaphysics)

Eligibility:
Citizenship:
Candidates who are joining the ISRO need to be Indian citizens.

Age limit:
Candidate must have minimum 21 years of age.

Qualification:
Candidates need to be Graduates from a recognized board or university.

Here I am giving you some B. Tech Astrophysics colleges for doing this course:

Indian Institute of Technology Bombay (IIT Bombay)
Indian Institute of Technology Kanpur (IIT Kanpur)
Indian Institute of Technology Kharagpur (IIT KGP)
Indian Institute of Technology Madras (IIT Madras)
PEC University of Technology
Bengal Engineering and Science University
Manipal Institute of Technology, Karnataka
Amrita Institute of Technology and Sciences - Tamil Nadu
Sathyabama University
Acharya Institute of Technology
Amrita School of Engineering
M.V.J. College of Engineering