Understand Church Music
Whatever organ you can play, play church songs on it for practice. The more you practice the more you will learn about the church music. Singing carols and other music in church with complete focus requires a great amount of practice.
Once you have decided to become a church organist, start attending the church services regularly. This will help you know the rhythm of the church music and that how the service is structured. For example, to start with you will need to play a song that gets the audience to join in the services with their respective responses.
When you will start attending church services regularly, you will get to know how the services are structured, how the music flows around the mass or services and how the pastor or father gets along with the organist even without congregation noticing it.
Speak to Organist
You can also speak to an organist who is already working a church. They can advise you better how you can plan out to work with the church. It is a better way to learn how you can plan your singing in the church.
While working with the church you must understand that you need a time commitment. You will need to observe punctuality, as the church cannot afford to delay its service. Since it is difficult to arrange an organist without being informed in advance, there can be serious troubles for you to continue working with the church.
Once you get into the church as an organist, learn more songs that are played at church services. This will help you grow in your skills. You can also talk to the priest and organizer of the church, they can help you in learning more songs. They can also give other useful tips.