If you're a singer, your voice is your instrument. If you're a choir director, the voices of all the choir members are your instrument.
To use those voices most effectively, you need to know what they're capable of, what their strengths and weaknesses are.
Listen to yourself sing. How do you sound at quiet volumes? At loud volumes? Some people are good at getting a sweet, gentle sound with their voice; others are good at getting a forceful or brassy sound.
How high can you go before your voice starts straining? Go to a piano and find out the names of the notes that are your top note and your bottom note (there's an online keyboard that you can use for this, too: http://www.music.vt.edu/musicdictionary/appendix/pitch/pitch.html). Be aware also, that with practice you can increase your range, so your top note today doesn't have to be your top note forever.
Directors should know this information about their choir members, too. Know your singers' strengths and weaknesses. Know what their top and bottom notes are. Then arrange your music so that it's a good fit for the singers in your choir. In the choir at my home church, the tenors can hit an F, but it's a challenge. Hitting a G is a struggle. I don't try to take them above that G.
A couple of things to keep in mind about choir music:
- In *contemporary gospel* choir music, the parts are often written in a way that leaves the tenors singing a lot of high notes, including notes that go above the standard tenor range, AND there are no baritone or bass parts. A lot of songs will also have numerous key changes, sometimes going amazingly (ridiculously?) high. The gospel artists who make the albums can pick and choose the most exceptional singers to be in their choirs, but in our churches, we're working with regular voices.
Knowing this, you may want to change the key or rearrange the parts on some songs to make things work better for your choir. And consider adding a baritone or bass part, so that men with bass voices aren't forced to either sing high tenor or leave the choir.
- In *classical* choral music, the pieces are written for people with trained voices, and trained voices can hit higher notes and hold them longer than untrained voices can. If you work with untrained singers, keep that in mind when selecting music. Try to use music that works with the vocal ranges of your members. With long passages that don't leave room for a breath, make sure your singers know how to work together and do alternate breathing (taking turns pausing for a breath while everyone else continues singing).
The better you know your instrument(s), the better use you can make of them, with beautiful results.
(By the way, ChoirParts.com will happily do parts on request in whatever key the customer wants. I recently did an order for a song in two different keys, neither of which was the key from the original recording.)