First thing you have to build you self confidence .for cracking any exam you have to be confident in yourself. 

Second thing is you have to be determinant that you can do it and will have to do hard work for acheinving success .

For cracking jee mains, you can read NCERT book specially for mathematics and chemistry. 

In chemistry you can read physical  chemistry and organic chemistry from NCERT books because as from previous year papers of jee mains, it is observed that many question is asked directly from NCERT book of chemistry. So for theory in chemistry NCERT is good option 

You can solve mathematics question from NCERT books of 11th and 12 th.

And for physics you can buy HC Verma book  for clearing you concept. By this book you can clear you concept of jee advance also. 

Thank you.