You have four months with you .
You can achieve anything.
Try to keep patience because it's all about to do regular studies with keeping patience and confidence.
Don't loose hope .
Because you have enough 4 months .
Try to increase your study hours forget about all and determined about your study.
First you should cover
Physical and inorganic chemistry.read theory again and again.
In mathematics you should start with algebra than geometry and than calculus
In physics you should start with 12 th .
Good luck .
Don't think that you weak.
Don't loose hope because it is not compulsory that if you learn everything than you can crack it
You can crack it with less syllabus but with proper learning and practicing.
Good luck.

Hello Sonu,

You should keep on revising your subjects to crack NIT. Thats the best thing to do. If you have joined coaching classes well and good, you can revise your materials from there. If not then solve NCERT books as much as you can. Focus more on chemistry it will lead you more marks and in less input. And attempt previous years papers. 

Good Luck.

If you have more doubts let us know.