Hello Siddanth,

Getting into organisations like NASA, ISRO and SpaceX is like a dream come true for anyone. Securing position in these organisations requires years of Knowledge, Practice and experience. You have to very very strong and clear at concepts with your respective subjects. Appear for national and international level olympiads and competitions held by these organisations. You should have degree from institutions like IISC, IIT's, BITS in order to get selected for interviews. You have to work very very hard in order to make your dream come true. 

All the best !