If you decide to go the variable route, as Andar suggested, I made a tutorial on that some time ago.
It can be found
here if you want to take a peek. It runs you through how to create the currency and how to manipulate it so simulate a custom currency.
You can then use plugins like
SRD_GoldWindowCustomizer and / or
SRD_MenuStatusCustomizer to display the variable values to the player. Of course you can also just use text-codes to display the variables. That might be less flashy though.
Depending on how you want to use the currency, a plugin might be favorable.
However, if you just want a basic currency, like money, exp, crafting material etc., it is easily doable with variables alone and can be done with my tutorial.