If you return an enchantment creature card with bestow to the battlefield, you can't pay its bestow cost. It won't be an Aura and can't be attached to a creature.
If the enchantment card is an Aura, you choose a legal player or object for that Aura to enchant as it enters the battlefield. This doesn't target the player or object, so it's possible to enchant an opposing creature with hexproof this way, for example. If there's no player or object the Aura can legally enchant, it stays in the graveyard.