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.

The bringing of the creature onto the battlefield and then putting Necromancy on it is all done as part of the resolution.

Necromancy enters as an enchantment and then becomes an Enchant Creature Aura as a triggered ability upon entering. It follows all the rules for Auras from then on.

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.

The sacrifice occurs only if you cast it using its own ability. If you cast it using some other effect (for example, if another effect allowed you to cast it as though it had flash), then it won't be sacrificed.