You choose the target for the triggered ability when the ability is put onto the stack. You choose whether or not Artisan of Forms becomes a copy of that creature when the ability resolves.

Artisan of Forms copies the printed values of the creature plus any copy effects that have been applied to it. It won't copy any other effects that have changed that creature's power, toughness, color, and so on. Artisan of Forms won't copy any counters on the creature, but Artisan of Forms will retain any counters it already had on it.

If Artisan of Forms becomes a copy of a token creature, it copies the original characteristics of that token as defined by the effect that put it onto the battlefield. It won't become a token creature.

The copy effect lasts indefinitely. Often, it will last until it is overwritten by another copy effect (if it copies another creature on a future turn, perhaps).

If the creature is an illegal target when the ability tries to resolve, the ability won't resolve. Artisan of Forms won't become a copy of that creature. It remains whatever it was a copy of.

When Artisan of Forms becomes a copy of a creature, it's neither entering nor leaving the battlefield. Any enters-the-battlefield or leaves-the-battlefield abilities won't trigger.

If another creature becomes a copy of Artisan of Forms, it will become a copy of whatever Artisan of Forms is currently copying (if anything), plus it will have the triggered ability.

Heroic abilities will resolve before the spell that caused them to trigger.

Heroic abilities will trigger only once per spell, even if that spell targets the creature with the heroic ability multiple times.

Heroic abilities won't trigger when a copy of a spell is created on the stack or when a spell's targets are changed to include a creature with a heroic ability.