How did / do we (the people) decide which decisions are made at the federal level, and which are made at the state level?

Having a mild debate on Facebook. There is a petition going around to get the President to legalize marijuana in Kansas. I’m trying to explain that it’s up to the states, not the president (“But Obama can do it if he wants to!” No, he can’t.) but I don’t understand it well enough to really explain it.