Learning which currency pairs to trade will usually depend on where you live in the world. For example, in Forex trading , if you were to live in the United Kingdom, you are most likely to find another currency to trade against GBP.
This is likely to be the USD or EUR, as these are the largest currencies in the world. The currency pair GBP/USD is probably the most common traded in the UK.
It is important to establish the pairs you feel most comfortable with so that you can gain experience in the way that they behave. You will need to follow the economic news relating to both currencies and there are numerous types of charts available for you to compare trends and patterns over set periods of time.
Just because you live within a specific country, this does not mean that you have to trade pairs that include the currency of your home nation. You may prefer the USD/EUR combination, or how about a trade that involves one of the strongest economies in the world at the moment - the AUD (Australian dollar).
As with anything else in life, over time you will decide on the currency trading pairs that work best for you. Just ensure that you follow the news that relates to the economies involved: e.g. employment news, GDP statistics etc. This has a huge impact on how rates will move - but this will all come to you with experience.
