You can choose the pair you want to trade, but the most people trade the main currencies, Euro, Dollar, British Pound, Japan Yen. And you can only cho

You may have already understood it, currencies are traded by pairs. The european Euro versus the US Dollar, The US Dollar versus the Japan Yen, etc. When you buy a currency, you want to sell it later at a higher price. When you sell a currency, you want to buy it later at a lower price. This is how you make profit. Think like you were buying a foreign company share. You always want to buy low, and always want to sell high.
What you are looking to when trading currencies is the exchange rate. This will tell you your next move. Buy or sell. Currencies are part of the economy of each countries. When the value of a currency is increasing, this means the economy is going better as before. The exchange rate can be viewed as the country's economy compared to another economy. This is why economic factors can help you to predict your next move. If you know that a currency will increase, you will buy it and expect to sell it at a higher price, a higher rate.
You can choose the pair you want to trade, but the most people trade the main currencies, Euro, Dollar, British Pound, Japan Yen. And you can only choose to trade one pair only if you want. You are the only person that will make the decision. Hope you are making the good ones, profit can be huge, as well as losses.
Like any business, forex trading has to be taken seriously. Lots of people are trading the forex and some are earning thousands of dollars every day. But it needs a lot of training, education and analysis before reaching such results. It can be the perfect business and actually it is for advanced traders