Hello Aditya,

In order to get admission in the top NIT college, you have to score in between 150 to 200 marks in the JEE Main 2019 examination.

In order to achieve this goal, the following tips could be very helpful for you-

The main thing is that you have to build your concentration level, then only you can stay focused on your goal. I would like to say that please limit the study time between 10 to 12 hours and please increase your concentration and speed of reading, understanding and solving the problems then you will not face any kind of difficulty.
For preparation of exam, the most important thing is your confidence after that you should know the whole syllabus and make your plan properly for learning new chapters, solving the numericals and revision. All three things should be the part of your preparation strategy in daily routine.
Your basics in Physics, Chemistry & Mathematics should be very clear upto 12th std. You should read the NCERT books for basics. Then HC Verma for Physics, RD Sharma for Mathematics, OP Tendon for Physical & Inorganic Chemistry, and Organic Chemistry of "Arihant Publication" for detail or extra knowledge. 
You should also try to solve the previous years question papers, this helps too much and you should also give the mock test to rectify yourself and build your confidence level.
For more preparation tips, importants topics & books, you can go through the link given below
Https://engineering.careers360.com/articles/jee-main-2019
In this way, you can prepare well for the JEE Main and the JEE Advanced.
Thanks for query on this platform, you may visit again
All the best for your examination.

You can check what rank You need to get to go to a particular college, Because there are more than 30 NITs and more than 20 Branches, also there is one state quota and other is Category quota so You can check Opening closing ranks at the JoSAA website the link is given Below:

https://josaa.nic.in/josaaorcrarchieve/Result/OpeningClosingRankArchieveN.aspx?Year=2017