you do mean making a brick (call it A) and a button that when pressed makes brick A fire, right? If so;
1)Create your turret brick, and button brick
2)Name your turret brick, Turret (or whatever you wish to name it)
3)event the button to say;
On Activate-->Named Brick-->Fire Projectile-->[][][]
Turret Gun Bullet
That should do it for a turret, but if you want one that tracks the player, you need an Add-On, which im afraid you will need to search for