M.Tech

I am afraid but you are not eligible for M.Tech. You must complete post graduation course before applying for M.Tech. Since you have completed Bachelor of Science, it would be necessary that you also complete Master of Science to apply for M.Tech course.
Master of Technology is post graduation course which is taken after completing Bachelor of Technology. The B.Sc students have two options to apply for M.Tech. You can either complete M.Sc as I mentioned earlier.
Other option is doing B.Tech through lateral entry. Just like diploma holders, B.Sc graduates are also eligible for lateral entry into B.Tech. You will get admission in the second year which would make B.Tech a three years graduation course for you.
Most of the universities in India will allow you admission through lateral entry scheme. The only expection would be NIT and IIT. For lateral entry admission, you must appear for Lateral Entry Engineering Test (LEET).
The other option as I mentioned is M.Sc. However you can also apply for MCA and then apply for M.Tech. Although this is not something that is recommended. You can get great jobs after M.Sc and MCA. This makes no sense to apply for the M.Tech.
I would say that only if you fail to find good job with M.Sc or MCA degree that you should apply for the M.Tech. Otherwise it is not recommended.
To get admission in M.Tech you will have to appear for General Aptitude Test for Engineering (GATE). It does not matter whether you have completed B.Tech, M.Sc or MCA, you must clear GATE entrance exam for M.Tech admission.
As far as branches as concerned, since your subject for B.Sc was Computer Science, you must stick to it. Either complete M.Sc in Computer Science or Master of Computer Applications. Regardless of the course you select between the two, you can apply for M.Tech in Computer Science through GATE entrance exam.