Civil Servant (IAS/PCS)

Firstly u decide over in which field Your knowledge is good. Or go for aptitude test. Then only u can opt Your field. Because IAS is related to Administration field. After completing of IAS u may join Higher Govt. post like Commisdioner, Personnel Secretary of Ministers, and so on.
And wheres BCA is a IT field. U can become software engr. and can do job or can conduct Your own business.

Both the streams are equally good. But most of the people opting for IAS or preparing for IAS go for social Science as it becomes easy to prepare for the paper for you get more time to spend for preparation. This is somewhat difficult if you are going Science subjects.
Enroll for graduation in some good university that offers you good library facility to prepare for IAS.

If IAS is ur passion go for it, check on google about it u will b informed in full detail, however u need to b a graduate first. So all d best

Please refer to following link for detailed process on joining Indian Navy after B.Tech: http://nausena-bharti.nic.in/eligible.php

Complete Graduation. Enroll yourself in some Institution who offers rigorous Coaching for IAS