Hey harsh!!

First of all , don't be disheartened. There is always a way and reason behind every thing.There is nothing to worry about. Just boost yourself for a new journey of 12th standard with enthusiasm leaving past behind. I have personally seen many students, who didn't do well or failed in 11th standard but came out as stars in 12th. You just need to study and right guidance.

As syllabus is entirely different in 12th from 11th except some portions where you need knowledge of 11th. I would suggest you to join some kind of coaching or any teacher for the guidance and to understand concepts. Other than that i am mentioning some books from which you can easily prepare with good understanding.

Biology: NCERT is a must , other than that you can refer to Trueman's biology for better understanding or if you are able to gather any notes that will be perfect.

Chemistry: again NCERT is must, you can refer to pardeep's publications book for preparation.

Physics: Numericals of NCERT is very important, for the theory you can refer to S.L. arora.

Sample papers: you can gather any book like arihant's publication for previous year questions. 

From these you can easily crack your exams with flying colours. The things should be remember are study, hard work, and consistency. This is all you need to prepare for your exams,

Hope it helps,

if any query , please feel free to comment.

Good luck.