Watch the Day 1 online guitar lesson by Ravi from Learn Guitar in 21 Days
All songs are written in keys, and the same song can be played in twelve different ones! However, generally certain keys lend themselves to certain instruments, and perhaps most relevant, a singer's range will determine the best key for his or her vocal.
Remember that the goal of this course is to quickly enable you to play guitar and sing along on your own or with your friends.
Playing along with your favorite recordings might be difficult if the original version is in a different key and uses chords you don't yet know, but that will surely change as you become more proficient.
For now, think about the songs in your head and sing the melodies, then the chords given in the song list should simply fall into place.