I’d suggest you to start revising your 11th and 12th standard’s concepts from NCERT. Revise these concepts thoroughly and practice a good amount of questions till you think you’re strong in all 3 subjects (PCM).
After this, start practising BITSAT past papers. They will give you a feel of the real exam and you can know where you stand. Start working on your weak chapters and concepts. The Arihant BITSAT book is pretty good for BITSAT. You can study the English and Logical Reasoning part from this as this is the most scoring part and can fetch you marks easily and quickly.
Keep practising papers in the end. Try attempting all questions if possible, in the given time limit. The questions asked in BITSAT are slightly higher in difficulty than JEE Main, but all of them aren’t tough. There are many easy ones also, so try doing them quickly so that you get more time for the tougher ones.
Best of luck!