Hello Shivam,

May be as you say because ALLEN is good institute but you should not depends upon that only. You should read NCERT books of Physics, Chemistry & Biology of class 11th & 12th at least 2 or 3 times for basics, your basics should be very clear upto class 12th. After that HC Verma for Physics (part -1&2), Biology by Trueman (volume-1&2) and Pradeep Guide on Biology, OP Tendon for Physical & Inorganic Chemistry, and Organic Chemistry of "Arihant Publication" for detail or extra knowledge. You can also read more books whose names are given in the link given below

Https://medicine.careers360.com/articles/best-books-for-neet
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 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.
To know more about preparation tips, syllabus, important topics, prevoius year questions with the solution, chapter wise weightage & the best books, you may visit the link given below
Https://medicine.careers360.com/articles/neet-2019
https://medicine.careers360.com/articles/neet-chapter-wise-weightage-and-important-topics
In this way you will be able to prepare yourself well and score a good marks in the NEET exam. 
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All the best for your examination.