The key of C is usually the first key covered in any music study. It doesn’t have any accidental meaning there are no sharps or flats, only natural notes. Therefore in the key of C, the notes are C D E F G A and B. The order of chords follow the same rule as any other major chord which is I, IV and V being major chords, ii, iii and vi being the minor chords and vii being the diminished one. The chords in the key of C are as following:

I (C)

ii (Dm)

iii (Em)

IV (F)

V (G)

vi (Am)

And

vii (Bdim)