shouldn't this be in the game design section.
But to answer your question, its simple(im asuming you are using gamemaker). Use a variable for the trigger being unactivates then have an object that when you collide with have the trigger activate, and with the object you want to do something. Put in something like if your variable is activated then do certain action
Example Code:
Create Event of Trigger Object:
global.trigger = false
Collison with Player Event:
global.trigger = true
Activated object's step event:
if global.trigger = true {//action goes here}
If you don't know how to code then use variable drag and drop boxes instead.
Make sure the trigger object is invisible.[/color]