B.Sc Maths

You should study well. Don't exert yourself too much. You should prepare for the exams in a smart way. Smart study is preferred.

You can pursue M.Sc followed by Ph.D. You can also make your career in the research field.

It is compulsory to take a general subject in B.Sc (mathematics).
Yes, you can take physics.

Mathematics has different branches like applied mathematics, pure mathematics, statistics and many other. You can choose any one of them.

Yes, it's a good choice. DIT-Dehradun is one of the best universities in India.

You can go for M.Sc from a famous institute like IIT or ISI. You can also learn higher mathematics for research.

Differential calculus by Spivak, integral calculus by Paul Dawkins Notes, Vector calculus by M.J.Strawn, and also abstract algebra by Hirstein.
Also, you can follow linear algebra by S.H. Friedberg.
Good luck.

Computer field came from mathematics. You can say that computer is a subset of mathematics. No doubt, you can pursue MCA from a recognized college or university after B.Sc in maths. You will get admission in the first year of MCA. You will have good job opportunities in IT and software development industry. IT sector offers a good salary package and excellent working environment for their employees. You study hard as jobs are waiting for you.
God bless.