One of the most important tools you can use to decode and understand music notation as a singer is the circle of fifths. In order to sight-sing accurately, you must be able to find a song's resting tone. In other words, you need to know which note is "do". Once you've done that, all other notes are defined by their relationship to that note.
To Find "Do":
Find the key signature in your song (immediately to the right of the clef). How many sharps or flats does it have?
Begin at the top (12 o'clock) position on the circle of fifths.
if the key signature has sharps, count clockwise one position for each sharp in the key signature.
if the key signature has flats, count counterclockwise one position for each flat in the key signature.
if the key signature has neither flats nor sharps, do not move in either direction.
The note name on the outside of the circle in the position where you stopped is "Do". If there are two notes in that position, choose the one that matches the color of the side you chose (blue for flats, red for sharps)
For any song in MAJOR, that "Do" is the resting tone. If the song is in MINOR, then look at the note on the inside of the circle that corresponds to "La", which is the resting tone in minor.
To determine which notes should be flat/sharp in a given key signature:
Find the desired key on the circle of fifths.
Use the process outlined above to determine how many flats or sharps should be in the key signature.
Look below the circle of fifths and find the box labeled "order of flats" and "order of sharps"
if your key signature has sharps, start at the right-hand side of the box and count towards the left until you reach the number of sharps in your key signature. All the notes you land on should be sharp.
if your key signature has flats, start at the left-hand side of the box and count towards the right until you reach the number of flats in your key signature. All the notes you land on should be flat.
if your key signature has neither flats nor sharps, it will have no notes that need to be changed.