See there can be a quite number of ways to get into ISRO.

• You can be ISRO scientist by qualifying the ICRB exam after your BTech.
• You can be ISRO scientist by taking admission in IIST which will make you eligible to get a job in ISRO after the completion of your BTech directly. IIST is the abbreviation Indian Institute of Space Science & Technology. You can get admission in IIST by qualifying JEE Advanced Examination.

I will say if you have good preparation then try for the second one to fulfill your dream of being a scientist at ISRO.

I hope this answer helps. All the very best for your future endeavors!