What am I suppose to prefer if I want to do something in the field of space after my 10+2 completion? And which institutes should I prefer if I want to study in India?

It seems that you want to become an astronomer. 
 
To be an astronomer, you need to take physics and astronomy classes at high school level. Also you need to get into advanced courses. At college level, take calculus, trigonometry and thereafter do major in physics or astronomy. 
 
To be an astronomer, it takes 8 years in college in which 4  years will be spent in getting bachelor's degree and then two years to get Master's degree and the last two years to get your Doctorate degree. 
 
Wish you success in your efforts
 
Some of the colleges and institutes offering course in astronomy are:

• Aryabhatta Research Institute of Observational Sciences (Nainital) 
• Indian Institute of Astrophysics (Bangalore)
• National Centre for Radio Astrophysics (Pune)
• Indian Institute of Science Department of Physics (Bangalore)
• Osmania University, College of Science (Hyderabad) 
• Punjabi University (Patiala)
• Raman Research Institute (Bangalore) 
• JNTU College of Engineering (Hyderabad)
• Andhra University (Visakhapatnam) 
•  Birla Institute of Technology & Sciences (Pilani)