It is really a good thing that you want to work with ISRO and I truly appreciate. Being a computer science graduate, You have a bright future and a huge scope in the ISRO. You can work as a Junior Research fellow.
The criteria required is first class with 65% aggregate. There should not be any backlog or gap for more than 1 yr.
ISRO conducts written test every year to recruit students from technical background for the post of Assistant Engineer post.
For more detailed information related to that, I would suggest you to visit ISRO’s official website i.e ISRO – Government of India
Prepare yourself for the exam. You can get sample papers on the internet.
All the best!!!