Most choral groups require their members to sight-read music. Sight reading requires the ability to look at an unfamiliar piece of written music and sing the notes assigned to the singer's part. It calls for a knowledge of musical notation and a good ear for rhythm and intervals. As with any other art, learning to sight-read is a matter of practice and study.
Learn the music staff for your voice part. Female singers and male tenors read from the treble clef, which has a curly symbol that looks like an "&" sign. The names of the lines are E-G-B-D-F from bottom to top, and the spaces are F-A-C-E. Male baritone and bass voices read from the bass clef, which has a symbol that looks like a backward C. Its lines are G-B-D-F-A from bottom to top, and the spaces are A-C-E-G.
Learn standard rhythm notation. An open oval equals four beats. An open oval with a stem equals two beats. A closed oval with a stem equals one beat. Every flag on the stem of a closed oval cuts the length of the note in half: one flag makes a half beat, two flags makes a quarter beat.
Learn the standard intervals. This is the difference in pitch between two notes. The most common training for this involves associating intervals with the first two notes of well-known songs.
Find sheet music for songs you are familiar with. Read along as you sing a song, either by yourself or along with a recording. This will reinforce the meaning of the symbols and make intervals more familiar.