CBSE has reduced the syllabus from Class 9 to 12 by 30% as the academic year of 2020-21 will also be reduced. Now the syllabus of BITSAT is almost as same as the syllabus of class XI and XII. As CBSE has reduced the syllabus so it will have a impact on the syllabus of BITSAT 2021 also. However, the authority has not yet announced. Once the authority announces the syllabus officially, you can start your preparation. For now, you must stick to NCERT textbooks rather than buying any other books. Also prepare for English Language and Logical Reasoning. Wren and Martin, Pearson BITSAT Guide are some best books for preparation. Go through last few years' question papers of BITSAT, You'll get an idea about what type of questions are usually asked. Check this
http://engineering.careers360.com/articles/bitsat-sample-papers