Dear Lavanyasaravanan,

It is good that you are appearing for so many entrance exam.

In order to start your preparation you can make your proper time table include all the topics for all the exams it will help you to complete your syllabus on time and you will also get proper time for revision.

You should work on your concepts. You can take help of NCERT books to build your fundamentals strong. NCERT will help you to prepare for JEE Main, BITSAT,VITEEE and for KCET you can refer your state text books. 

Refer previous year papers to understand the exam pattern and to know the level of the exams.

Solve as many as mock test series for your practice.  Time mangement is a key to score good marks make sure you work upon it. 

JEE Main exam level is high then other exam so if you are well prepared for JEE Main you can easily crack other exams.

All the Best!!