Image source : google
Every year Isro conduct examination for entry level scientists/engineer SC , just clear written then a interview and you will be a scientist in Isro (group A officer) .
Since it is reaserch orientated organisation so you must have very firm grasp on technical part. They don't simply asked you definitions in interview but they are more interested to know the physical significance behind principals.
The other way to join Isro is to crack gate (under AIR 2000) examination and then get admission in iist (indian institute of space technology) for masters program. It will surely increase your chances to crack interview. Because your thoughts is going to resonate with ISRO's vision.