you did not mention which course you want to join in iits, however for students who are studying in 12th course science stream, may join iits b.tech program by clearing jee main and jee advance exam, apart from btech course few iits has bachelor in design course i.e b.des which admission is offered through CEED EXAM,
for students who are in final year of bsc course may join iits by clearing IIT JAM exam,
for final year btech students or students who have already passed btech course may join for mtech course, or m.s course in iits by clearing GATE EXAM,
also there is option for master degree holders to join phd program in iits.
hope it would help.
Admission in IITs is provided through marks or rank obtained by you in JEE Advanced examination so if you want to get into IIT , you need to appear for JEE advanced and clear it , now to be eligible for JEE advanced it is mandatory to qualify JEE mains first .
Thus you need to clear both JEE mains and advanced examination if you want to get into IIT.
Also, to be eligible for admission in IITs you must have got at least 75% marks in class 12 if belong to general or obc category or at least 65% marks if belongs to SC/ST or must be in the top 20 percentile in class 12 examination.