Dear Aspirant,

If you want to become an astronaut then you must try for air force as many ISRO astronauts are from Air Force. For that you should focus on science now and then Physics Chemistry and Mathematics and Computer Science in your XI and XII.

To join air force, you also have to be graduate first. I suggest you should go with engineering, preferably Aeronautical engineering. You can then join AF and then you can go for ISRO also.

The exams you must prepare for are:-

JEE main and Advanced

AFCAT

Hope this helps. Thank you.