Scarcity and the economic problem
Every society faces the same basic problem: resources (land, labour, capital) are limited, but human wants are effectively unlimited. This forces every economy to make choices about what to produce, how to produce it, and for whom -- these three questions are the foundation of economics.
Opportunity cost
Because resources are scarce, choosing to use them one way means giving up the next best alternative use. This forgone alternative is the opportunity cost of the choice made.
Example 1: A farmer has one plot of land and can grow either yam (worth ₦200,000) or cassava (worth ₦150,000) on it, but not both. If she chooses to grow yam, what is her opportunity cost?
Her opportunity cost is the cassava she gave up growing, worth ₦150,000 -- the value of the next best alternative she sacrificed by choosing yam instead.
Economic systems
A capitalist (market) economy relies on private ownership and market forces (prices, supply and demand) to decide what's produced and how resources are allocated. A socialist (command) economy relies on government/state ownership and central planning to make these decisions. A mixed economy, which most real-world countries (including Nigeria) use, combines private enterprise with government intervention -- the state provides some services and regulates markets, while private businesses handle much of production.