To become a scientist at ISRO, you should have studied Mahematics and Physics at 10+2 level. You should first complete your 10+2 with Physics, Chemistry and Mathematics and then qualify in IIST. If you want to get into ISRO after Graduation, you should complete your Bachelors with a minimum aggrehate of 65% in any of the streams like Electrical, Electronics, Mechanical, Computer Science, etc. Because it is 2 years from now, we would suggest you to keep yourself updated about any changes in the process meanwhile.