Hello aniket,

Hope you are doing well.

As per your question, as you didn't mention after what you are going to choose isro like after 12th or degree . But you can join isro either passing 12th or after completing your bachelors degree in any field of engineering.

After passing 12th you can persue any cosmology, or aerospace field from the colleges like IISC or from IITs and nits as you will be directly recruited from their if you have relevant skills.

Or you can give isro exams after completing your bachelors degree from any of the branches which is mandatory that you should have your degree in btech or BE. After clearing your written paper they will call you for Personal interview (PI)  which would be tough enough to confuse your brain. But they check each and every concepts knowledge in you and the way you answers.

After shortlisting they normally have a payscale of 56,100rs per month which is fixed for CLASS C category.

For more Information regarding ISRO you can visit :-

https://www.google.com/amp/s/engineering.careers360.com/articles/isro-recruitment/amp

Hope this information helps you out. Thank you