There are no instructions that I know of (IIRC there is an Encyclopedia entry but it only shows a picture).
The usual keys apply for starting the engine and for movement. I think the turret auto-seeks enemies and locks on, then you press the [TAB] key to fire. Where there are multiple enemies I found that I could use the mouse to rotate the turret toward the one I wanted to kill first.
The turret doesn't rotate through 360 degrees, so I found the best way to use the gun when being pursued was to drive in reverse and hope you don't run into an anomaly!