The best option for you is to excellent in JEE Advanced.Assuming your from the general category a rank below 3500 would land you a seat in Indian Institute of Space and Technology Trivandrum.From where your chance of getting inducted into ISRO is very high.

ISRO did campus recruitments this year from very few selected colleges. My college of NIT Calicut was among them.But this need not be trend that ISRO might follow in the coming years and it was for ECE and mechanical branch.So i guess IIST is the most safest option if you are very determined to join ISRO.

ISRO also conducts an exam to induct you as a scientist.You can clear that exam to get a job as scientist at ISRO.

Hope i was helpful.