You already know that dairy products should generally be stored in the refrigerator, although I always keep some butter on the counter in a closed butter dish for spreading on bread and adding to a hot pan.
Cheese is best stored loosely wrapped in the fridge instead of shrink-wrapped in plastic as it normally comes. I often stash cheeses in an airtight container to keep their strong smells contained, then take them out before serving to let them warm up a bit. If a soft cheese develops mold, you'll probably want to throw it out. If a hard cheese develops mold, just remove it (plus a bit extra) and eat the rest (sooner rather than later).
I generally believe in the smell test for evaluating milk (does it smell gross? don't drink it!) and the Save The Food campaign by the National Resources Defense Council backs me up on the legitimacy of this method. They also suggest using sour milk in lots of recipes, which we'll explore here soon.
Pro Tip: buy yogurt plain instead of flavored and you'll have multiple ways to use it up. You can add syrup or jam or honey or fresh fruit when you want to eat it sweet, but you'll also be able to use it in savory preparations like Creamy Green Dip. I also make most of my pancakes with yogurt because I almost never buy buttermilk (yogurt, buttermilk and sour cream can often be used somewhat interchangeably, with slight adjustments for texture, acidity and salt).