If this creature would be dealt damage from multiple sources at the same time (say, because it's blocked by multiple creatures), all of the damage is prevented and only one +1/+1 counter is removed.

If damage that would be dealt to this creature can't be prevented, the damage is dealt and a +1/+1 counter is removed from this creature.

If this creature has no +1/+1 counters on it (but is still on the battlefield because something else is raising its toughness), damage that would be dealt to it is still prevented.

If this card is going to take damage from a black source, you can choose to apply Protection from Black's damage prevention ability prior to applying this card's built-in ability. This means you don't have to remove a counter.