This question is far too broad to be answerable with enough detail, and it depends on whether you mean the absolute bare minimum or whether you mean "to perform a particular function". Let's assume it's the bare minimum, which I will assume to mean "people can go there in their own spacecraft, stay for a while, and then leave, all safely".
This answer will be unsatisfying, since it will be very general and ignore many important aspects of the realities of space station design.
We can look at this generally. First, it requires some kind of habitable space, a module that the crew can live in. It requires life support, which at a basic level means temperature control and air. It requires electricity to run all of these things. It also needs docking port(s), where the visiting spacecraft can come and connect*. This docking port needs to have a way to transfer crew and supplies.
Assuming that the supplies (like food, air, water) are stored in the habitation module, and said module has the sleeping/eating/living/toilet spaces all in one, then at a very high level that's all you need.
Unfortunately, this isn't really sufficient - it glosses over decades of design iteration. How thick are the walls? What are they made of? Are there windows? (do there have to be?)
It also ignores things like how food is prepared - is it all protein bars or do the astronauts need to cook? How does plumbing work?
*OK, strictly speaking a visiting spacecraft doesn't need to dock if the astronauts are willing to...jump? However, no space station ever made lacked docking ports.