The first currency is the base currency. The second currency is the settlement currency.
E.g. for the USD/SGD pair:
If you wanted to obtain Singapore Dollars (SGD), you would SELL the USD/SGD pair.
If you wanted to obtain United State Dollars (USD), you would BUY the USD/SGD pair.
In forex trading and in brokers such as Interactive Brokers, this would be the way you’d convert from one currency to another.