To get admission in top IITs ands NITs for CSE branch, the closing ranks are below top 75 AIR.

Hence, it is advisable to score well above 305+ to 325, to definitely get CSE branch.

You need to focus on important topics and the weightage has been given beloe alongwith preparation tips:-

https://engineering.careers360.com/articles/jee-advanced-syllabus

https://engineering.careers360.com/articles/jee-advanced-preparation-tips-physics-chemistry-and-mathematics

Top books you can take up for refernce are as follows:-

Physics:- 

Concepts of Physics Vol I and II by H.C. Verma

.New Pattern JEE Problems: Physics by DC Pandey.

Chmeistry:-

Organic Chemistry by O.P. Tandon

Numerical Chemistry by P.Bahadur

Mathematics:-

Maths XI & XII by R.D. Sharma