Dear Dhananjay,

In order to start your preparation make sure you are aware of the complete syllabus and exam pattern.
You should go through previous year paper to understand the exam pattern and level of questions 
Go through NCERT books for your preparation. So, that you have good hold on your basics. 
If you are preparing for JEE Main you can easily crack BITSAT
BITSAT is a online test it is recommended to solve as many as mock test series for your practice and to analyze your preparation level at regular interval of time

Good Luck!!