If you get a seat at IIT, then You should report at the respective IIT on the date mentioned, There you will do final formalities of Registration Including Biometric verification,Fee Payment from the data of JEEAdvance at the same time You will be doing your course registration for semester one and you are done, this is what You need to do after getting a seat in IIT. Now you are a student and rest another life depends upon the rules established by your IIT.