You should first first to get your singers relaxed. According to Vocalist.org.uk, being relaxed is the first step toward proper singing technique and a good sound. Many choral directors implement a tactile approach to relaxation, having the members of the chorus hold hands or give each other gentle neck massages. This helps the chorus feel united, and less awkward or frightened to sing. When it comes time to actually sing, breath control is a good thing to address first, which is how much air you use to sing every note. You can have your chorus do deep breathing exercises, practicing awareness of their lungs and diaphragm. Another exercise involves blowing your breath out at a constant stream, timing your next inhale. This gets singers aware of how much air they are using, and when to breathe as they sing each note.
Regarding actual singing exercises, there are many options. Start off with scales and arpeggios. Scales are sets of eight consecutive notes on the keyboard. In the key of C, a scale would be C-D-E-F-G-A-B, repeating C as the 8th note at the end. Arpeggios are patterns derived from the first, third, fifth and eighth scale tones. For example, in the key of C, an arpeggio would be C-E-G-C. Another exercise that might help is singing, in harmony, the chords to the pieces that you will be playing. Chords are like arpeggios, except all the notes are sung at the same time instead of individually.
It is also crucial to develop the hearing ability of your singers. Not only does singing demand good breath technique and diction, you also must have well-tuned ears in regard to pitch and harmony. You can practice this by playing certain melodic or harmonic passages on a piano, having your singers identify it, trying to sing it afterward. With harmonies, you can have singers sing the root notes, or the first and lowest note of each chord, individually, or harmonize the chords together. It is necessary for singers to be able to hear what they will sing before they produce sound.
Sight reading also is a good skill to develop. By regularly practicing new music that your singers have never heard, you can train their ability to see, hear, and sing written material. As most chorus music is written out this way, developing reading ability can help you establish a large repertoire faster.