Hello applicant,

Well it is said to have atleast first class mater's degree in maths/chemistry/physics/animal science/cognitive science/agriculture science from a recognized university.

In engineering you can go for computer science/electronics or mechanical engineering and if you can get a decent score in gate.

Hope it helps.

Hello aspirant, 

Well to join DRDO, aerospace engineering is one of the best option to join as it has scope in future. Also, DRDO works in this stream. After joining aerospace engineering you can go for internship in DRDO, and according to your skills and interests you can later join the DRDO. 

Hope it helped to clear your delimma.