Hey 

Prepare your class 12th topics first as you will be appearing for your 12th boards as well. This will help you to score well in both Boards as well as in exams like JEE/BITSAT. While preparing topics make sure that you cover sufficient depth of the topic (i.e. try prev. year questions). 

All the Best!

Hey

Primarily concentrate on the topics that overlap with your 12th Boards syllabus. This will help you maintain a balance and prepare strategically. Make sure that you try prev. year questions (BITSAT/JEE) so that you have a fair idea of whether you are covering these topics from the competition point of view or not. 

All the best!