The first note you will learn to sing is called Middle C. Middle C is the note in the middle of the keyboard. Since this is a singing book and not a keyboard book, it is not important which finger you use to play middle C (or any other note).
The important thing is to listen to the pitch of the note and then copy it with your voice. Many people find this difficult at first, but it gets easier with practice. A good way to become confident of singing the correct pitches is to imagine the sound of each note before you sing it.