Hi, To prepare for BITSAT you can follow the below mentioned steps.

First, Go through the NCERT material. If not start with it soon. Other rerence material won't help you if you miss this important book. Also do not skip previous year papers . It is second most important thing to do. Practice them with or without answers. Next, work on your accuracy but mostly on your speed. SPEED is your support. In Chemistry study organic and inorganic reactions. They are the mains. In Physics try solving more sums than just focusing on tough ones. For English vocab you can get it by your own. read newspapers, novels and watch english news channels. It shall help you alot. So all depends on your practice, hard work and dedicaton. All the best.