I agree with fj102. Try to have a look at the history of money.
Basically, here are a few tips. Initially, there was no money. Everybody was bartering (know what barter means?). Gradually, people found it more convenient to use some common medium of exchange, say a piece of seashell, to facilitate exchanges of goods. Later, people made a few changes in this medium and settled on some type of precious metal, say gold or silver.
People found it difficult to carry gold or silver around and started to deposit gold or silver into something called a bank in exchange for a certificate, which they could use to buy goods with. Later, the government nationalized to privilage of issuing these pieces of paper.
But these pieces of paper could still exchanged back for gold or silver. And later, the government found it more convenient to just declare that these pieces of paper are not redeemable, that is, not being able to exchanged back into gold or silver. And that is where we are now.
Got any clue? Good luck!