To join IIT you need  to crack JEE advanced, you will become eligible to appear for JEE advanced once you clear JEE main. the joint entrance examination main is conducted twice a year once in the month of January and another in the month of April  students who are in class 12th or already qualified the class 12th not more than two years ago are eligible to appear for JEE main ,they must have Physics and Mathematics as their subjects in class 12th. Around the country more than 10 lakh students appeared for JEE main out of which only two lec qualify for JEE advanced and out of 200000 appear for JEE advanced only top 10000 students are able to grab a seat in IIT, for IIIT the admission is offered on the basis of your performance in JEE main.  JEE advanced exams are based  on the syllabus of class 12th of physics chemistry and mathematics but the difficulty level is far beyond the board level exams NCERT,  in class 12th you can apply for JEE main January edition in the month of September and for April edition in the month of February.