If you aren't careful, you will get bitten with the DDR bug
As a quick song deal, you can start out in the free play mode, being able to select any song you've unlocked, at any of the few difficulties.
If you want an idea of "beating" the game, you can always hit the dance master mode. Here you follow along a path of predetermined songs and modes/difficulties until you beat them all. After about 60% of them, the game gets exponentially harder.
If you're looking to lose weight, hit the workout mode and see just how many calories you've lost.
Dancing to songs gets you points which allows you to unlock items by going to the shop. This could be anywhere from new tunes, to different skins for the characters display on screen while you dance.