Teach them that borrowing money obligates them to pay the loan back with interest. And that credit cards are set up so that they keep making payments each month without ever paying off the debt. In fact, if they pay the minimum due on a charge card each month, it's just like doubling the price of everything they buy. That's a lesson that's less painful if you learn it before the bills come due. Teach them what things cost.
Some families share budget information with their teens.
Others prefer to keep that private. If so, send your teen on a pretend 'first apartment' hunt. Have them walk through all the costs of setting up an apartment including rent, utilities and food. It will be a real eye opener for them. Finally, help them to learn the difference between creative thinking and creative financing.