they will fire depending on the mode automatically, they just need to be tied to an active firegroup as i mentioned originally. If it's not on forward fire, you having it set to trigger 1 and pushing that button wont cause them to fire. They'll fire according to the mode they're in.
If you have fire at my target, you have to at least fire once at the target before they will start firing. If you switch to a firegroup where no trigger is assigned to the turret, it wont fire.
If you have it set to fire at will I believe it waits until your ship is fired upon before shooting an enemy. I forget though as I always use fire at my target mode.