Cows need space, feed, water, and a place to sleep. What you feed it and how depends on the type of "cow." Since the term "cow" is always used very loosely and can refer to any type of domestic bovine, no matter if it's a young calf or an older bull or feeder steer, then the act of caring for a "cow" is varied depending on that type of bovine you are referring to. How you care for a bovine also depends on where you live and what you intend on doing with that bovine.
Ultimately, health records should be kept, proper feed and grazing practices should be done, and a constant supply of water and mineral is what is needed to care for that "cow."