Sight-singing or what most people think of as “reading music” is the most important skill a choral singer can possess. Sight-singing is best learned and practiced using Solfeggio or Solfege (soul fedge), with movable Do (dough) — also known as “Do-Re-Mi, etc.”
This system applies each of the generic syllables to a particular tone in the scale. Therefore, if you are singing a piece in the Key of C, the note C will always be “Do” and the D note will always be “Re”, etc. If you sing in the Key of G, G will be Do, but C will be Fa, or in other words, movable Do.
This makes it possible to think in the same paradigm no matter what key or actual notes you are singing. After a singer practices this method for a short time, and his/her ear becomes more trained, it becomes easy to sing melodies without accompaniment and confidence increases with the mastery.
For composers, solfege is indispensable; it allows one to write down a melody without having an external instrument at hand . . . only the mind’s ear, pencil & paper are necessary! There are a number of excellent Internet resources for sight-singing & ear-training that allow the student to practice and ultimately master this important craft. However, nothing can truly replace a skilled instructor to fine-tune one’s skill level.