Hello dear

You can choose any course for the eligibility of the cds all you need is your degree from a recognize university or Institute . And if you done your graduation in engineering than you will be able to join air force and navy also but if you are not from engineering field than you can only opt IMA or OTA.

So if you have science ( maths ) in 12 th go  in the btech field which will provide you all the options .

Good luck