re: 40 songs everyone has heard, The same 4 chords, the same progression.

All you have to do is sit by the radio with a guitar and you will be amazed at how many songs you "already know". And if you change the order of the four chords, say G-Em-C-D instead of G-D-Em-C, you increase your repertoire tenfold.

G-Em-C-D is affectionately known as the "Dead Teenager" pattern.

Here's how it's done