Can I get a good job in defense sector after doing my BE in computer science?

Yes, absolutely. You can work in almost every government job after B.Tech and for Indian Defense. Be free to share any problem you have.

Yes, you can apply or CDS AFCAT or any other such exam for army recruitment. Similar to this you can also find computer related career in other defence allied departments like IB. There are very less number of vacancies offered for particularly computers and IT but still there are opportunities.