When a song consists of notes from a particular scale, it is said to be written in the key which has the same notes as that scale. For example, if a song contains mostly notes from the C major scale, it is said to be in the key of C major.
The following song is written in the key of C major. The key of a song indicates how high or low you have to sing. As well as singing the lyrics, it is beneficial to practice the melody using sol-fa syllables, as this will help you to learn the pitches of the notes accurately.