Hello Praveen

Th only way to achieve this dream of yours is to join the IAF that's the best possible way for you and if they think you are capable they will let you fly Jet aircraft so you will be ready for wars or any other situation and ISRO will soon be sending astronauts of their own and they will be probably be choosing the best pilots from the IAF so score good marks in exam and be  the best in physical fitness and always be the best wherever you are in IAF only that way the IAF will recommend you for being an astronaut in IAF

hope this helps you