firstly i want to advice give more concentration on your NCERT books as you are in 8th only.
Also NCERT books are very good for faundation.
In iit more attention is on concept clear bcoz questions are conceptual tough.
So first clear your concept in each class(8th,9th,10th,11th,12th).
As well as refer a book named FOUNDATION OF IIT JEE IN CLASS 9th & 10th.
In your leisure hour make your basic strong by the help of this book.
Class 8th is the best time to start preparation for iit to crack iit in first chance by <100 rank.
GUD LUCK NAWAZISH
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