On the Survival Server in The Economy Update we implemented a shop/economy system. This would allow players to create shops and exchange goods and services.
To obtain currency to use in the economy you will need to find Diamonds and mine them. Once done so, use
/sell Diamond or
/sell hand and you'll be notified that your balance has been updated.
You may find that you want to obtain your Diamonds back from your balance to use. To do this you will need to go to the Survival spawn and look for the Red Tent. From there you can find an Admin Shop setup where you can buy back Diamonds from your balance.