The Axis of Awesome: 4 Chords Official Music Video
You can apply the I, V, vi, IV chord sequence to any key. The vi is a minor chord. In the key of C the chords are C, G, A minor and F. You don't have to stick to that order though. Try Am, F, C, G, to see what pop songs come to your mind.
For a different key, write out the notes of that major scale to work out the first (root), fifth, sixth (minor) and fourth chords. e.g. In the key of G, the first chord is G, the fifth is D, the sixth minor is Em and the fourth is C. Or you can write it as follows:
I V vi IV
G D Em C
Very friendly chords for beginner guitarists! Ukulele players may prefer to stay in the key of C to begin with. This formula works in any key though.