If you're teaching a song that's not recorded (or it's not recorded as a choir song), your chords are a good starting point. The different vocal parts together will be forming a chord, and they will follow the chord patterns of the song.
If you're teaching a song based on a recording, practice listen to the recording carefully and trying to sing along with each part in turn. If you can master that and memorize the parts, then you'll be ready to teach the parts to the choir.
When you're starting out as a director, you'll spend a lot of time practicing this skill. I used to sing choir parts out loud constantly when I was at home alone and while I was driving. The more you do it, the better you'll get.
God bless you in your new ministry!
