You have just now passed 12th. It is very obvious to think about the future at this stage of life. You have already decided to pursue B.Sc in Computer Science.
As per your query, it seems that your final goal is either to pursue M.Sc. in Physics or M.Tech. Please have a look at the below suggestions that would help you in opting the correct course.
1. After B.Sc. (Computer Science), you can pursue M.Sc. (Com. Sc.). This way you can make yourself strong in your core field which will be very fruitful for your career. In today's era it is very important to be educated in the same field to get good job.
2 There is no issue doing M.Sc (Physics) after B.Sc. (Comp.Sc), but the career opportunities after this will not be like after M.Sc. (Comp.Sc). Hope you understand this .
3. Now, After B.Sc. (Comp.Sc.), opting M.Sc. (Physics) and again pursuing M.Tech will be time taking. You can do even more better without doing double PG.
4. If your final goal is to pursue M.Sc (Physics) then you better opt B.Sc. (Physics). After this you can prepare for UGC NET examination to build your career in teaching. You can do better in core field.
5. If your goal is to pursue M.Tech you can do B.Tech in Computer Science. As per your query, seems you are interested in Computer Science so good to pursue M.tech (Comp.Sc.). This can done either from private engineering collage or from Government engineering collage. You will have to score good in GATE examination after B.Tech to get the government engineering colleges like IITs or NITs or State Universities.
Hope the above comments will help and guide you in opting the correct course. Wish you all the very best for your future.