Hi Navjodh, there is no need to panic if you haven't done well in graduation but from now onwards you need to be more cautious for the upcoming choices and coming back to your question there is no minimum marks(graduation)  criteria you just have to go through the admission notification for IITs. Every IIT has their own criteria for example IIT Delhi demands 70% in B.Tech for Gen/OBC .

So if you want to get into the IITs filter out the IIT that suits you

And for NITs just go through the CCMT notification

There are so many colleges you can get in to.