Hi tanmay,
Firstly I think you are not clear with the selection criteria to get into nit or iit  . For getting into nit one must score atleast 75 percentage in his or her class 12 th exmination according to maharastra hsc borad if he or she belongs to general category or obc category but if you bolelong to sc or st caste then scoring 65 percentage in 12 th board is sufficient to
Clear the first step to take admission into nit or iit .
And top 20 percentile was about the percentage students score in 12 board of their respective state as every staes have their own cutoff percentage according to toughness level.

Once you clear 12th with cutoff percentage now you can easily apply for jee exam if you qualify jee exam with good score of atleast 170 to 180 then you can think to get into some respectable nits with good branch now a days there is percentile system in jee exam so you must have to score atleast 88 percentile to qualify jee mains .   Now once you clear jee mains to get into iits you have to clear jee advance to get into iits iits take admission through jee advance for which you have to clear the two initial steps of 75 percentage in 12 th and clearing jee mains .

Hope u get my point !!!!