Hello Tarun,

The first step is that you should complete your bachelor's degree be it B.E. or B.Sc. or B.Tech. in a college.

If you want to pursue engineering, you will have to write the JEE Mains, and also qualify for JEE Advance and hope you make it into the IIST rank list. 

Then take up a good course in IIST  if you have a good CGPA throughout, you'll be absorbed into ISRO.

Another option is to complete B.E./B.Tech from any other AICTE approved college in India,  give ICRB's exam for Scientist/Engineer for your stream, clear the interview and get into ISRO.