This is a very basic tutorial how to make a trigger. For example, i've choosen to make a func_breakable entity in a wall, and i want to trigger it to break it by itself.
So, just make a normal room. When you want to make a triggered func_breakable entity, make a smaller room behind the wall where you want the entity to be at. So, then, make another object, then move it to that wall. Then, you can make it look more like a "randomly" broke wall. Then, after you move it inside the wall, make it thick as the wall is. Then, press Ctrl+Alt+C to carve it. Then, choose the texture which the wall has, then put it in the hole in the wall. Then, while you have it selected, click on "To entity".
While in the properties menu, select a func_breakable entity. You'll see a lot of options. Then, select the "Name" option, then name it as you wish.
Then, select an "aaatrigger" texture and make an object that is expanded from wall to wall. Then, click on "To entity". Select a trigger_once entity and select the "Target" option and type the name of the func_breakable entity.
And there you got a triggered breakable wall entity!