Every budding musician is likely to follow a certain process in learning about music. First he will take piano lessons at a young age. Then, in elementary school, he is likely to take up an instrument, violin or trumpet, for example. He will learn what fingerings or keys go with which notes on the printed music, so that he will be able to sight read on his instrument. But if he's involved in choir, chances are that his notes will be demonstrated on the piano, and he'll learn his line that way.
The skills of vocal sight reading, or sight singing, are not usually addressed until the student reaches college, and then only if he's a music major or minor. Is this because the skills involved in sight singing are highly advanced musical skills? This has often been considered to be the case. Most courses in sight singing assume a high level of musical knowledge on the part of the student.
Is this assumption valid? I would claim that it's not. In fact, the skills involved in instrumental sight reading are considerably more advanced than those involved in sight singing. When a student plays the piano or violin, he has to know which notes are flatted or sharped, so that he can play the appropriate notes.
When a singer undertakes to sight-sing a musical line, he really doesn't need to know about flats and sharps in most cases. He needs to know how to find which note on the staff represents the tonic note, or "Do". Then he needs to get used to how the other notes in the scale sound in relation to "Do". The key signature is used to identify "Do". But then he can forget all about flats, sharps, keys, and clefs, and just sing the notes in relation to "Do".
The sound of "Do" in any song can be identified by considering the end of a song. Virtually all songs in western music have the melody ending on the tonic note. A way of demonstrating how the other notes of the scale sound in relation to this tonic is to give examples of songs that start on certain notes of the scale. A song that starts on "Do" will begin on the same note that it ended on. An example is "My Country, 'Tis of Thee". If you were to sing the last phrase, "let freedom ring", and then start the song again, "My country... ", you'd find that it began on the same pitch it ended on, the tonic note.
Most songs, however, do not start on "Do", but on another note of the scale. The relationship between these notes can be demonstrated by finishing the song and then starting again. Sample songs that can be used include: (Re) Yesterday, (Mi) Three Blind Mice, (Sol) Here Comes the Bride or London Bridge Is Falling Down, (La) I Dream of Jeannie with the Light Brown Hair, and (Ti) O Danny Boy. By considering these songs and other similar ones, a budding musician can learn how each note sounds in relation to the tonic. This will give him the ability to sight sing with confidence, even if he is young or has not yet mastered the playing of an instrument.