There's nothing very formal about it, and most choirs will be very happy to have you join them. It's best if you know or find out the name of the minister or choir master. The minister is addressed as 'dear Reverend , the choir master simply as 'dear Mr. . You then simply state that you would very much like to join the choir, you state your age and whether or not you have any prior musical education and/or experience (and what kind) and whether you can read music a little bit - or even with some ease (they like it if you can, but otherwise they'll teach you). If you have any idea yourself, tell them what kind of voice you have. Depending on how high or low you can sing, that's either soprano, mezzo or alto if you're female; and tenor, baritone or bass singer if you're male. Don't forget to write how they can best reach you.
