How do you find the key of a chord?
The way to figure out what chords are in a key is to look at each note in the key and use a pattern to figure out each chord. The first note, C, gives us the C Major chord. The second note, D, gives us the D minor chord.
What key is my song?
The easiest way to figure out the key of a song is by using its key signature. The number of sharps/flats in the key signature tell you the key of the song. A key signature with no sharps or flats is the key of C (or A minor).