You do not necessarily need any formal training to become a church musician, though some sort of skill or talent in playing musical instruments is preferred. Options for complete amateurs are available as well, such as singing in a choir, but to become a church musician who plays a musical instrument, you will first need to learn how to play that instrument. Piano and organ are perhaps the most common instruments played at churches, though guitarists are also common. Other instruments that are less common are not necessarily precluded from church playing, either, so do not get discouraged if you play an instrument not commonly played at church.
If you are already proficient at playing a particular instrument and have the desire to become a church musician, approach the music director at your church, if one exists. Volunteer your services for masses, special events such as weddings or baptisms, and so on. It is likely that the music director will work with you if you are eager to learn and are willing to sacrifice your time to learn the songs, though some music directors may not be able to accommodate the musical instrument you play. Do not get discouraged; try to find ways to volunteer your services and perhaps develop a plan for incorporating your instrument into the musical scheme of the services.
Singers and pianists are usually in high demand, so if you want to become a church musician and are not picky about what instrument you are playing, consider learning the piano or developing your voice. While some church choir groups will teach you how to sing, others may not, so you may need to take voice lessons to learn how to sing properly. If the church offers lessons, attend choir practices and learn the songs as quickly as possible. Remember that plenty of practice will be necessary to become a church musician, so budget your time wisely and appropriate enough time during your week toward practicing with the choir and on your own.
Learning to play the piano can take a long time, and you may need to take years of lessons before you are prepared to become a church musician. Even if you already know how to play the piano, you will still need to be prepared to play in front of the entire congregation, and you will need to learn the parts of the mass so you know when to play and when to be silent. Attend church regularly and take note of the musical pieces, as well as when they are played.