To get admission in B.Tech or B.E, only PCM subjects are mandatory so if you wish to get seat in top NITs then you can follow below provided preparation tips -

1. Know your Syllabus Well: Before starting on the preparation process, there is one important step to be completed and that is you should the syllabus properly.

2. Work on a Time Table: After carefully checking the syllabus of JEE Main 2018, work towards creating a perfect timetable which will fit your schedule while leaving enough time to prepare for the examination within the days left.

3. Make your Basic Concepts Strong: While it is essential to study the topics mentioned in the official syllabus, it is also advised to study the basic concepts well first.

4. Improve your Weak Points: By constantly studying the topics you find difficult, you will be able to be well prepared in them for the upcoming examination.

5. Write Notes and Prepare Flash Cards: Keeping notes greatly helps during the preparation process.

6. Attempt Mock Test and Solve Sample Papers: Attempting mock tests and solving sample papers is perhaps the most popular strategy to prepare for any examination, and it is rightly so. Mock tests will give you an almost real experience of how the examination will be.

7. Revision: While studying it is important to revise those topics which has already been studied. It is advised that each day, a certain amount of time should be divided for revision purposes.

8. Make Sure to Take Breaks: Between the stressful studies, it is important to take short breaks to refresh the mind.

9. Check out Proper Books for Preparation: During the preparation period, you need to make sure that the proper books are being referred and studied from.

10. But most importantly, BE DETERMINED: One thing to keep in mind is that all of these tips and preparation methods will not work if you do not have a determined mindset.

Books for Jee main which you can refer -

Physics -

Concepts of Physics Vol I and II by H.C. Verma Problems in General Physics by I.E. Irodov Fundamentals of Physics by Halliday, Resnick & Walker.

Chemistry -

Numerical Chemistry by P. Bahadur Organic Chemistry by Paula Bruice Yurkanis Inorganic Chemistry by J.D. Lee Organic Chemistry by Morrison & Boyd.

Mathematics -

Maths XI & XII by NCERT Trigonometry by S. L. Loney Co-ordinate Geometry by S. L. Loney Higher Algebra by Hall & Knight Problems in Calculus of One Variable by I.A. Maron

Hope this helps!!!