Yes, first of all, you will need to give JEE Mains exam and for that you will need to have atleast 75% in the 12th board examination then after giving exam if you clear the cut-off of JEE mains exam then only you will be eligible to give JEE Advanced examination which is conducted for admission in IITs. After giving JEE Advanced exam if you clear the cut-off in your category then you will be eligible to sit in JoSAA counselling for IITs. But clearing the cut-off is never sufficient enough to get admission you need to score much higher than the cut-off in order to get a better rank and admission in an IIT.

So, if you don't score 75% in 12th board or atleast in top 20 percentile among the students who have appeared for board exam with you then you won't be eligible for JEE Mains and without clearing JEE mains you can't give JEE Advanced and get into an IIT.

Feel free to ask if you have any more queries. All the best!!