Instruct the choir to sing a single melody in unison. Pick a melody that doesn't go higher than a D in the staff or lower than an E on the lowest line of the treble clef. Men will sing an octave lower than women. Use the piano to help the singers ensure that they are singing on the correct pitch.
Have the women sing the original melody, and instruct the men to sing a major third below the women. Play the men's part on the piano while the choir sings. Then play just the women's part.
Sing a chorale as a warm-up every day. The chorale should have four separate parts. Teach each part to the choir individual. Start with the sopranos and play along with the piano, then move to the alto, tenor, baritone and bass. Baritone and bass generally will sing the same part, separated by an octave.
Practice chord-building exercises. On the piano, play the root of a chord and have the basses and baritones hold that note. Then, add the tenors a major third higher, the altos a minor third higher than the tenors, and the sopranos should play the root above the altos. This will build the major chord C E G. Once the chord has been established, cut the choir off and have them sing the chord again. Do this for major and minor chords.