Glad that you have set your eyes on Astronomy. While you are finishing year 12, appear for JEE,NEST and if you are lucky get into IITs, if not you can get into BSc at a top Physics or Mathematics University and then focus of IIT-JAM to try for IIT once more or a M.Sc. India's list of colleges that offer good physics and astronomy colleges,
1. Indian Institute of Technology, Bombay(IIT-B)
2. Tata Institute of Fundamental Research, Mumbai (TIFR)
3. University of Delhi (DU)
4. Indian Institute of Science, Bangalore (IISc)
5.Indian Institute of Technology, Delhi (IIT-D)
6. Indian Institute of Technology, Madras(IIT-M)
7.Indian Institute of Technology, Kharagpur (IIT-K)
8. Banaras Hindu University, Varanasi (BHU)
9.Indian Institute of Astrophysics, Bengaluru
10.Institute of Mathematical Sciences, Chennai (IMC)
After this, you can consider courses like the 1 year JAP, Joint astronomy program wherein you get a chance to do research under eminent faculties and also leading on to a PhD. This would certainly open your doors to get access to publish, present and interact with national and international authorities.
Also, try to locate an astronomy association in your city or a city nearby. For example, in Banglore, there is the Association of Bangalore Amateur Astronomers (ABAA) who conducts summer classes, practical astronomy courses etc. The best way to find out is going to the city's large planetarium and finding out more. There are many amateur clubs (www.astrosociety.org,https://www.britastro.org/) around the world that you can register for free astronomy material and you can be on the look out for scholarships and young talent opportunities.