Introduce yourself to your church's choir director. Ask him if he is currently accepting new members, or when he will be looking for new members. Some church choirs only accept new members in September and January, as that's when they start to learn new music for Christmas and Easter.
Find out if he requires an audition, which he may do just to place you in the proper section (soprano, alto, tenor or bass). Tell him about your musical background, including whether you play an instrument.
Ask when and where rehearsals are held. Be sure you are available on their regular rehearsal night and that you are ready to make this commitment before you agree to be a part of the group. Also ask if there are special performances around the holiday, requiring extra rehearsal time.
Attend your first rehearsal. Introduce yourself to the people sitting in your section to help ease your nerves a little. They are likely to be happy to add another member to the choir. Pay close attention to the choir director's cues and most importantly, enjoy yourself.