hey, don’t worry. From now onwards you can prepare from now onwards. The questions of Physics, Chemistry and Maths are generally from the academic level of senior secondary. Yeah some higher level questions are also asked. I can suggest you some good books for your help. Like 

Physics

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

I.E. Irodov – Problems in General Physics

Halliday, Resnick & Walker – Fundamentals of Physics 

Chemistry

NCERT – Chemistry XI & XII

P. Bahadur – Physical Chemistry

Arihant Prakashan– Organic Chemistry

O.P. Agarwal– IIT Chemistry

IIT CHEMISTRY ,Arihant Publication

Maths

R. S. Agarwal – Maths XI & XII

Best Book and Must Do --RD Sharma

S.L. Loni – Co-ordinate Geometry

Hall & Knight – Higher Algebra

MCQ Maths: Das Gupta

Guides and helps books of Arihant Publication.

Proper strategy ,time management and hard work is the base of success in any exam.Well if you want to prepare your self for engineering exams and want some guidance then you can join Online IIT Coaching which will provide you the best online tutorial related to IIT and AIEEE Exams at your home.This will help you to make your strategy for exam and would complete your syllabus on time.You just need to join the site http://www.etoosindia.com and make you able to access all the material needed for preparation.

If you want to pass AIEEE and BITSAT exams, it is most imported for you should be hard work, time spend on study, and join any coaching center. then you can pass your exams.