You've to start preparing for the entrance examination right from now. You need to clear your joint entrance examination that is the JEE. The exam will have the three main subjects physics, mathematics and the third subjects as per the latest rules can be of your choice out of the available choices. Earlier it was chemistry.  Once you do this, you'll need to clear the NASA entranxe examination held by them to get eligible to pursue and complete your graduation of astronaut over there.

There are various criterias that you need to meet alongside as well like maximum blood pressure 140/90. Height range: Between 62 and 75 inches for Astronaut Pilots and between 58.5 and 76 inches for Mission Specialists Distant visual acuity: At least 20/100 or better uncorrected that is correctable to 20/20 each eye. You also need to pass the psychological screening test.

You also have another option of choosing ISRO.

Best wishes. Thank you.