How can I strengthen my basics concepts?

First, you can approach a good tutor, which can give you proper knowledge according to NCERT books. There is no need to read any extra book. NCERT is a bit boring, but you need to go through it. You can also practice previous year paper.

Try solving variety of questions from reference books like H.C.Verma (Physics), D.C. Agrawal (Maths). Keep revising the concepts with questions. Also take a look at previous year questions of IIT exam. This will help you a lot.