What should i do if i drop a knife into my foot?

It depends on the result. If no visible injury, of course no need to do anything; and a small cut probably doesn't need anything but a band aid. If you have a large cut or uncertain, go to an urgent care clinic or emergency room to have it checked.

It has to do with your immediate symptoms, if open wound, stopping the bleeding would be first. At a minimum you suffered a contusion that will cause a bruise and pain that may last few days. A fracture will be suggested by severe persisting pain and seeking medical attention will be best.