how should i study for bits? any references?

I would suggest you to go through the below points -

1. Know your Syllabus Well.
2. Work on a Time Table.
3. Make your Basic Concepts Strong.
4. Improve your Weak Points.
5. Write Notes and Prepare Flash Cards.
6. Attempt Mock Test and Solve Sample Papers.
7. Revision.
8. Make Sure to Take Breaks.
9. Check out Proper Books for Preparation.

Reference books which you can follow -

Physics
Concepts of Physics by H. C Verma Vol 1 and Vol 2
Problems in General Physics by I.E. Irodov
Arihant Physics by D. C. Pandey
Advanced level Physics by Nelson and Parker
Fundamentals of Physics Resnick, Halliday, Walker

Chemistry
Atkins' Physical Chemistry
Physical Chemistry by N Awasthi
Organic Chemistry by Paula Bruice Yurkanis
Organic Chemistry by M.S. Chuahan
Inorganic Chemistry by J.D.Lee
Inorganic Chemistry by O.P.Tandon

Mathematics
Higher Algebra by Hall Knight+ Bernard-child
Differential Calculus by Arihant
Integral Calculus by Arihant
Problems in Calculus of One Variable Algebra by I.A.Maron
Class XI & XII by R.D. Sharma

(a) English Proficiency
(b) Logical Reasoning
BITSAT English Proficiency & Logical Reasoning by Disha Experts
The Pearson Complete Guide to the BITSAT (English) by One Learn Education
Wren and Martin.

Hope this helps!!!!