Once you decide what key you want to play the song in, that becomes the "1". All of the other numbers follow the notes in the scale for that key. The 6 will usually be flagged as "m" because the 6 is the relative minor of the 1 (the key).
rotoiti is correct; those whould be the chords for the key of G.
If you want to play it in C, then: C D E F G Am 1 2 3 4 5 6m.
"Just because you've always done it that way doesn't mean it's not incredibly stupid." - Despair, Inc.