In Equestria. Exchange of goods and service uses something called Bits as the medium. 

It is understood that there are also such thing as credit, it is entirely possible that in major city there are such a thing as credit card or tender. There might also be taps. Depends on how familiar you are to other characters, they might allow you to pay back later or even borrows.

Bits look like coins, gold coins to be exact. Gems stone are not known to be an acceptable exchange medium considers how easy it is to find. However they might still worth mony. There are currently no known paper currency in Equestria, but it is entirely possible that there are such a thing called Cheque or Banking system in other major cities.

To get bits, pony work for a wages. A lot of places will hire pony. Both canon and non canon location