First of all from your question, it seems that you have wasted your precious time and now you are feeling that you are lacking behind by your friends.
1.How should you study.
First of all you know your situation very well so plan your study accordingly and keep some points in your mind
1.star your study with that subject which you enjoy most
2.make your body habitual of sitting for atleast 2-3 hours a day
3.clear all the basic concepts of physics and chemistry and practice maths seriously
4.avoid wasting your time with friends and on mobile
5.set your goal which motivate you towards your study
6. Have a healthy diet (the most important thing)