Bards are one of the few classes that have such a large set of features unique to themselves. Those being songs. Songs are learnt from repetition. Originally the first bards had to learn them from non-player character bards throughout the realm, but they can also learn from each other.
The more complicated the song the longer it will take to learn, but once a bard has heard a song for about a dozen times they should be able to sing it properly afterwords. However, whether the outcome of the song has any effect depends on the bard's other nature skills.