Read NCERT textbooks of Physics, Chemistry and Biology of 11th and 12th standards.

Give equal attention to every subject as they are all equally important.

Solve Multiple Choice Questions after you finish each chapter of each subject. 

Solve as many numericals as possible in Physics. Understand concept behind it because if concepts are clear then any question can be solved. 

For chemistry also you must practice numericals of mole concept, thermodynamics, Chemical kinetics, etc. Practice organic reactions as there are many of them and you need to practice them all so that you can remember them well. 

Refer reference books like HC Verma Volume 1 and 2 for Physics,  Modern Approach to Chemical Calculations by RC Mukherjee, Concise Inorganic Chemistry by JD Lee ,Objective Biology by Dinesh, Pradeep Guide on Biology, etc. 

If you still have any queries feel free to ask in the comment section down below.