For getting admission in IIT, you should score a total aggregate of 75% in your 12th examination. Your marks in Physics, Chemistry, Mathematics, language (if you have taken more than one, the higher scoring one will be considered), and a subject that is not one of the above mentioned. 

The marks scored in your 12th exam is given more preference. However, for appearing in entrance exams, knowledge in the chapters taught in your 11th will be of help in gaining a good score. So, it will be helpful if you revise and have a basic knowledge of what was taught to you in that period if you are unable to completely cover that portion.