You don't need any special preparation for bitsat if you are preparing for jee. You just need to focus a bit on reasoning and english otherwise the syllabus for both bits and jee mains is the same also the level of bitsat paper is easier as compared to jee main but yes if you are only focused on your 12th boards then start revising your class 11 syllabus and try to solve a variety of questions and increase your problem solving capacity.