First off welcome to LGM, YOU ARE WELCOME, my suggestion is similar to Formuzik's teach the melody line first that way their is a base to harmonize to. If you try to teach the harmony part firsrt you will find it a little harder to teach the remaining parts. The most important thing you can do is practice, practice, practice, until you are sure of all the parts. Choir parts are usually chords corresponding to the music played along so, if you play piano, use the chords to find where the parts are. For an example "Sound of Praise" Bishop Larry Trotter, for the part that goes "Jesus loves to hear the sound of Praise", the parts would be:
Je sus loves to hear the sound of Praise
Sopranos: Ab G F Ab G G Ab G Ab which is the melody
Altos: Eb Eb Eb Eb Eb Eb Eb Eb Eb which makes it pretty
Tenors: C C C C Bb C Db Bb C which gives completes the sound
Let me know if this helps at all!
Peace!