Find the Community guide and read the scripting tutorials. This type of thing is covered in the first and second tutorial referencing light switches... just relate that and imagine that instead of having the Light's "If Used" field set to the light, it's set to a spawnable entity.
Find out what script a triggerzone uses to spawn an entity and mold that (if necessary) into a switch script.
The one and only,
Those who live in the past, are destined to insanity. Those who live only for the future, will be slaves to their ambitions.
Those who live in the moment... only they, are truly happy.