How should I store a potato after it is cooked? Foil, Tupperware?

This depends on the cooking method. If baked for example, you can wrap the potato in Saran wrap or foil, or in a Tupperware type container. Same for other methods, mashed, au gratin, etc. can be stored in any bowl, plate or Tupperware. Just be sure to cool the food down to room temperature before covering. Store leftover potatoes in refrigerator and consume withing two days.