In order to start your preparation make sure you are aware of the complete syllabus and exam pattern.
You should go through previous year paper to understand the exam pattern and level of questions

Go through NCERT books for your preparation. So, that you have good hold on your basics.

If you are preparing for JEE Main you can easily crack BITSAT as syllabus is almost same only difference is that BITSAT have English Proficiency and Logical Reasoning section. You should read the newspaper daily to score good marks in this section.

BITSAT is a online test it is recommended to solve as many as mock test series for your practice and to analyze your preparation level at regular interval of time

Below are the important topics which you should focus more as it carries more weightage in the exam

Physics

Mechanics (Newton’s Laws, Work and Energy, Rotational Dynamics etc), Thermodynamics , Electrostatics and Electromagnetism.

Mathematics

Integral Calculus, Differential Calculus, Coordinate Geometry and Complex Numbers.

Chemistry

Ionic Equilibrium, Chemical Equilibrium and Gas Laws

All the Best for you Preparation!!