Hello nitin,

For getting admission in good college you need to get good marks in JEE mains. But should also focus on 12 board exam. You can do both the things simultaneously. If you haven't started preparation for JEE then you should probably drop a year and should focus mainly on your boards.

Make a daily basis time table which should cover all of your subjects i.e physics, chemistry and maths. Give more time to the subject which is tough,

At night do a quick revision of what you have covered and what else is remaining. 

Try to solve numericals and practice papers on weekends.

This way it will be easy for you to prepare for your 12 board exam easily. 

Hope this will help