Hey, 

With 68 percentile, you cannot get into any of the NITs even after considering your reservation. If your score is or above the cut off marks of your respective category, you can proceed to give mains. To enter the IITs, you'll have to clear advanced then, you'll be approached for the counselling. Few private engineering colleges admit students based on the JEE mains score and other colleges require you to write the entrance examination of the respective college. 

Hope this helped you 

Thank you and keep asking :)