Draw your fingers across the lute, listen to the song, play from the heart, unleash your soul.
Musicianship is the bases of the bardic skill set. Without the knowledge to play a bard would fail at even the simpliest of endevers.
Musicianship itself has little use without one of the other bardic skills. Used in conjunction with Discordance, Peacemaking, and Provocation, your musicianship will decide if you fail before even attempting the skills.
It is always recommend to train any skill from an appropriate NPC before attempting to master it. For bardic training seek out your cities local conservatory of music.
Once you have attained a novice level of skill in Musicianship simply playing any instrument will give you a chance to enhance your skill. Local bards would love to provide eager musicians with new instruments. Be forewarned, each instrument has a limited number of uses. No bard worth their songs would be caught without a few backups.