To take admission in IITs you should have 75% marks in your boards exam or you should be in top 20 percentile. So, if you have 75% marks in your boards you don't need to give improvement exam. 

Good luck!!

To take admission in IITs, NITs. IIITs,GFTIs you should have minimum 75% in class 12th. So this marks is sufficient to apply there is no requirement for improvement exam. You can check the eligibility criteria with given below link.

JEE Main 2018

You can also check the detail about JEE Main application form.

JEE main Application form

Good Luck!