Have a trigger zone around the turret and in OnTriggerEnter() check if user has pushed your Use button. If they have, make the player invisible (GameObject.visible?) by using player.gameObject.active = false and then switching the current Camera off and the Turret's Camera on. Then you need to write a script to control the turret

Edit: To make a trigger zone, go to GameObject -> Create Other -> Cube, resize the cube until it is the right size for your trigger zone, then on the box collider check Is Trigger and remove the Mesh Renderer. Then tag your player as Player and then you can check if your player walks into the trigger zone like this:

function OnTriggerEnter(collision : Collider) {
    if (collision.gameObject.CompareTag("Player") {
       // Player has walked into trigger zone.
    }
}