There are four seasons: spring, summer, fall, and winter.
Each one has a pre-defined songlist. I can't remember offhand which has which songs.
Selecting a particular season in the "scroll game" will grant you a certain number of points towards unlocking the next song in that season. This is represented graphically by unrolling part of the scroll: once a picture is entirely unrolled, the song corresponding to that picture will be unlocked.
You don't need to worry about wasting points; once you've unlocked alll the songs in a particular season, its cards will disappear from view and you will not be able to select them.
Once you've unlocked all four seasons' worth of songs, all the cards will turn to "hell" - putting enough points into this unlocks the Hell expert course.
As has already been mentioned, certain songs are unlocked by having DDR STR!KE and GF/DM V saves on your memory card when starting PNM 12.
Also, one song is unlocked by completing the bicycle mini-game, and one is unlocked by completing the mountain mini-game. In the bicycle mini-game you try to pedal as fast as possible to the end of the level, jumping over any obstacles in your way and catching your cat if he's knocked out of your basket. Once you complete enough levels (3, IIRC) you get the unlock.
In the mountain mini-game you try to select the pose which matches the deity's in a short amount of time each turn. Once you complete enough poses (maybe 30 or 40) you get the unlock.
There are also 6 or 9 songs which must be unlocked via playing them (but not necessarily passing them?) in Pop'n Request mode.
I think that's everything useful (aside from a list of the songs themselves, which can probably be found on the VJArmy wiki). | 
