Great Ambition there.
Subjects to choose are Physics and Maths.
You can specialise on either, but you should be good on both.
First, learn physics and maths thoroughly.
You need to pass 10th and 12th with good marks , and then pass entrance exams to get into a college in india or abroad. If you can prove your excellence in college, you can easily pursue your dream.
So, begin now by learning well.
You also will need good english language knowledge.
The exact steps are not relevant. This is becaus , i only know how its done now. By the time you pass out of college, it would be 8 years or more. I cannot predict what will be condition then.
Now, your aim should be class 10 and 12 th, with good physics marths bvackground.
YOU CAN ALSO CHOOSE CHEMISTRY OR BIOLOGY.
But NASA is mainly physics and maths. You can be an astronomer, or a scientist in NASA