You have to make choices as to which goods and services your going to buy. This is where opportunity cost comes into the picture. Opportunity cost is the highest valued alternative to a choice you make. Since every choice has an alternative choice, opportunity cost is always present. For every good or service you purchase, you could always have chosen to buy a different good or service.
Society also faces an opportunity cost for each choice they make.
Every good or service that we as a society decide to create, means that the resources that produced that good are not available to produce other goods.