Hello Gagan!

You can do a lot of things to effectively killl your time. You can go through your college syllabus and get to know about what subjects are there and what can be difficult for you based on your interest now. As you will be pursuing BSc in Physics, you can go through fundamental topics of Physics in your +2 once again. On the other hand, you can read other books like novels that would inspire you and develop your knowledge

If you are interested to build your skills in computer applications or so, you can learn some programming languages like C,C++ etc. Also start using online resources and try to know about recent developments in the branch you choose. This will help you to have practical idea about topics and later helps in choosing project. There are MOOC courses online that are provided by leading universities of the world for beginner level students in every field. You can browse through such courses in platforms like Coursera, Edx, Udemy, etc. You will get a shareable certificate after you complete the course and that will be helpful when you apply for universities or jobs abroad(if you want to). I am attaching a link below for some free or paid courses related to physical sciences. Go through it if you are interested.

https://www.coursera.org/courses?query=physics

https://www.coursera.org/courses?query=coding

Copy and paste the above links in your browser.

If you are interested to learn new languages, then you can start right now! There are good apps and online courses like duolingo. If you have time, you can definitely give it a shot.

All the best!