These currency pairs are also represented by three-letter notations such as USD for US Dollars, EUR for the Euro, JPY for the Japanese Yen and so on. Among these currency pairs, you have to choose then where you want to invest. This decision, of course, will be based on your market analysis on what currency will go up on value for the next few days or months. You have to keep in mind however, that there are currency pairs that are very volatile - or those that fluctuate easily and may fluctuate steeply that can make you lose more or gain more abruptly.
Of course, if you are not that eran money prepared to face such market changes, it is wise to research and be well-informed before going to live trading. You can also invest your time on practicing in a demo account so you will see and feel what it is like to be trading.
To learn how to trade foreign currency, you have to understand also the factors that can bring the value of a currency down or up. You can do this by doing a forex market analysis which can be done in two methods - the technical analysis and the fundamental analysis.