Complex sentence: Verdi's gift for music was already apparent by 1820/1821 when he began his association with the local church, serving in the choir, acting as an altar boy for a while, and taking organ lessons.
Split into simpler sentences: Verdi's gift for music was already apparent by 1820/1821. At this time he also began his association with the local church, serving in the choir, acting an altar boy for a while, and taking organ lessons.