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.