In Forex, you deal with currency pairs. There are four main currency pairs: British Pound and USD (GBP/USD), Euro and USD (EUR/USD), USD and Japanese Yen (USD/JPY), USD and Swiss Frank (USD/CHF).
In each currency pair, the first currency works as commodity and the second one works as money. For example when you choose GBP/USD to trade, if you buy, you buy British Pound against USD and if you sell, you sell British Pound against USD. It doesn’t matter what currency you have in your account. The trading software takes care of the exchanges and transactions automatically