How should I prepare to get admission in the NITs or IITs?

You can prefer NCERT text books. You should revise regularly. You can also solve many MCQs. You should try to solve last year's JEE Mains papers. Best of luck.

To get admission in NITs and IITs, you need a strategy. You need to make your basics clear and rigid so that you can move forward. You can read NCERT books. You should practice as it will help you to understand the theories in a better way and also you will come across some unknown methods to solve