can i do b.ed after bca to become government teacher and which subject should i take for b.ed

No and B.ed is not required if you want to be a computer teacher. to teach traditional subject you should do major in that subject only Such as BA English, BA B.Sc Mathematics, BA History, BA Geography etc. There is no computer specialisation in B.Ed hence there is no seat for BCA graduates

Yes you can. Choose mathematics and english as your specialization as they are relevant to your degree. Do pursue mca also if possible.