When putting a card onto the battlefield that requires a definition for its value or some other choice, you do what is needed to define the value or make the choice.

This is a new wording. Animate Dead is now an Aura. You target a creature card in a graveyard when you cast it. It enters the battlefield attached to that card. Then it returns that card to the battlefield, and attaches itself to that card again (since the card is treated as a new object on the battlefield).

Once the creature is returned to the battlefield, Animate Dead can't be attached to anything other than it (unless Animate Dead somehow manages to put a different creature onto the battlefield). Attempting to move Animate Dead to another creature won't work.

If the creature card put onto the battlefield has Protection from Black (or anything that prevents this from legally being attached), this won't be able to attach to it. Then this will go to the graveyard as a State-Based Action, causing the creature to be sacrificed.

A "creature card" is any card with the type Creature, even if it has other types such as Artifact, Enchantment, or Land. Older cards of type Summon are also Creature cards.