Your store needs to have a base currency setup before it you can start selling. If you store theme allows multiple currency, you can add more currency and setup the exchange rates.
To setup select Settings -> Exchange Rate
Click the Enter New Store Currency.
Currency
Select the currency that you require.
Is Default Currency
Check if this is the base currency that will be used in the store.
Exchange Rate
The exchange rate ratio based on $1.00 of the base currency.