The basic movements are four: right, left, up and down, although sometimes you'll have to perform them together, exactly as if you were dancing in the dance floor.
Songs? Don't worry about songs, you'll find a lot of them in the official site. There are a lot of packs of songs submitted by the big and active community of users of Stepmania.
How to install Stepmania songs?
It's easy, download the pack of songs from the official site, and unpack the content of the file into the folder songs of the game.