From earlybeatlessongs:
Paul McCartney was born on June 18, 1942, and brought up in an orthodox working-class household with mum and dad, and brother Mike. His father, Jim, was once leader of a dance hall troupe called Jim Mac’s Band, and was the main source of McCartney’s early musical tutoring (such as it was). Not only did McCartney senior expose his eldest son to various forms of music, but he kept an upright piano in the front room, which McCartney readily learned his way around. (Jim Mac played trumpet too, which McCartney also took up in his youth.)
In 1953 McCartney passed exams for the Liverpool Institute, and a year later, a certain George Harrison did likewise, the two striking up a friendship through meetings on the bus and in the playground. Little did either of them realise that history was already in the making.
McCartney’s father had given him a trumpet for his 13th birthday (June 1955) but after he’d mastered the basics, he became bored with its restrictions (for one thing, it prevented him from singing while playing). Rather, McCartney gravitated towards the piano. In 1956, he seems to have come up with his first original songs on that instrument. Since this was prior to the arrival of rock and roll, and since his musical grounding was more towards music hall, these first efforts bore no real relation to rock music as we know it today, consisting initially of piano ditties. Two of them survived, and are considered to be the first two he ever wrote (although this is impossible to prove for sure).
Officially appears on McCartney - Archive Collection
Officially appears on Sgt. Pepper's Lonely Hearts Club Band (UK Mono)