When making low temperature cheeses, whey will spill out from pressing or hanging curds. However, it is probably easiest for most people use the whey from making kefir or yogurt cheese, or the soft cheese of the Fundamentals eCourse.
Pour the whey into a clean jar and store in the refrigerator for many weeks. Or freeze for many months. The whey will last a long time.
It is normal for a few milk solids to slip through the cheesecloth with the whey. Over time, they can get moldy floating at the top of the whey. Strain them off as necessary, and usually the whey is fine. It should smell fresh; let your nose be your guide.
Homemade Whey https://traditionalcookingschool.com/food-preparation/weekly-kitchen-tips/askwardee-uses-for-whey/