A ballad is a type of traditional folk song, extremely common in the British Isles. There was a concerted effort to begin collecting and writing them down in the 18th and 19th centuries, and many have been recorded and preserved. Many popular musicians continue to work in the ballad form.
Formally, a ballad consists of four line stanzas rhymed abcb, with the first and third lines being iambic tetrameter and the second and fourth iambic trimeter.
To compose a ballad, you need a story, perhaps an interesting event from the news. Another possibility is adapting one of Aesop's fables or some other traditional tale into ballad form.
The main trick to writing in the form is to have a very simple story line, with a limited number of characters, with each character being a clear type (the noble hero, evil villain, innocent maiden). Stereotypes and redundancy are much more acceptable in this sort of oral genre than in literary poetry.