When you play a melody on an instrument, each note has a specific fingering, so you can always check the pitch by playing that fingering. However, when you sing a melody, it is not possible to see the exact positions of the parts of your body which change the notes (the vocal cords), because they are inside your larynx.
For this reason it is a good idea to refer to a musical instrument any time you are unsure of the pitch of a note. In this book, all new notes are shown on a keyboard. If you have a keyboard you can play each new note and then sing it.
On any keyboard, the black keys always appear in groups of two or three. The C note is a white key. It is always on the left hand side of a group of two black keys. Find all the C notes on your keyboard.