Most people would really like it if they had the ability to pick up a sheet of music and know how to sing the notes accurately without hearing them demonstrated first. But we are typically given the impression, usually by musicians, that you must be a musician in order to learn sight singing. They tell us that you need some background in music theory or playing an instrument before you can really understand sight singing.
I would beg to differ. Sight singing seems intimidating because it's basically learning a new language. But as challenging as that metaphor seems, if a language had only twelve words, as music does, it would be far easier to learn. Here are three shortcuts that enable a person to learn the skills of sight singing without having any background in music.
The scale is the key (pun intended). Rather than trying to figure out which notes are sharped or flatted, you can just realize that the lines and spaces of the music staff correspond to the notes of a major scale. If you can sing a major scale (think "Joy to the World" or "The First Noel"), you can learn to sight-sing. All you need to do is to locate the starting note of the scale on the staff. There are some simple rules for that. Then the lines and spaces around it are all scale notes.
Another imaginary obstacle to sight singing that makes it seem exclusive to musicians is the clef sign. This is the symbol at the beginning of each line of music that tells which notes the lines and spaces correspond to. But the good news is that we can totally ignore the clef sign (in almost all cases). We can use the key signature, that little collection of sharps or flats at the beginning of the line, to identify the starting note of the scale. Then the other notes fall into place around it, as explained above.
A third imaginary hurdle to learning sight singing is the presence of accidentals in the music. There are so many different accidental signs; how can you keep them all straight? Actually, there are five different signs (sharps, flats, naturals, double sharps, and double flats) but only two functions. An accidental either raises a scale note by a semitone (the smallest interval), or it lowers a scale note by a semitone.
So which ones do what? It's really pretty easy. Sharps and double-sharps always raise the scale note. Flats and double-flats always lower it. The confusing one is the natural sign. It can either raise or lower it. But it's really not that hard because if there are sharps in the key signature, the natural sign lowers the scale note. As you can probably guess, if there are flats in the key signature, the natural sign raises the scale note.
Now that you have shortcuts around the three big obstacles to sight singing, the key, the clef, and accidentals, there's nothing (other than practice) to keep you from becoming as good of a sight-singer as you want to be, even if you don't know anything about music theory.