Hi Ravi!!

There is two eligibility criteria of 10+2 marks for having admission in any IIT  through JEE exams. The first is 75% and the other one is top 20 percentile in your board. If you are from CBSE board the top 20 percentile end upto 80%. But if you are from state boards like UP state board, Bihar board, etc. then you have chances to get an IIT even at 65 percentage. Good luck.

Hope this helps.

The marks you scored in your 12th exam doesn't matter much for your admission in IIT Kharagpur.  The only way to get admission in IIT Kharagpur is through the entrance exam that is jee main and advanced. This is an all India exam.   First you need to clear jee main and then advanced.  After clearing both of them and securing a good rank ,you can get admission in IIT Kharagpur or in other IITs. Hope this helps.