You should go for engineering to become a scientist in ISRO. Try to crack NITs and IITs and you can take degree in B.Tech in Aerospace Engineering, Mechanical Engineering, Radio Engineering, and Engineering Physics. To get selected in ISRO make sure you get good grades in your academics. 

After completing B.Tech from any college, an examination namely ICRB (centralized Recruitment Bangalore) is conducted by ISRO and individuals are tested as per their respective branches of engineering and after clearing the interview, one can get appointed as a scientist or an engineer at ISRO. Candidates must have a minimum aggregate percentage of 65% and above in B.tech in their respective relevant streams.

Good Luck!!