Oracle Text

Whenever you cast a creature spell, tap target creature.

Card Rulings

10/4/2004 Can target an already tapped creature.
10/4/2004 The ability is not optional. If you have the only targetable creatures, you have to tap one of your own.
4/1/2008 A “creature spell” is any spell with the type Creature, even if it has other types such as Artifact or Enchantment. Older cards of type Summon are also Creature spells.