If you put an Aura onto the battlefield this way, you choose what it enchants as it enters the battlefield. This doesn't target any permanent or player, but it must be able to enchant that permanent or player. For example, you could put a green Aura onto the battlefield enchanting a creature with hexproof controlled by an opponent, but not one with protection from green.

You don't have to put the card on the battlefield even if it's a permanent card. You may choose to leave it in exile and create a 1/1 blue Fish token.