Click in the game area with your mouse or hit the 'S' key to start. Use your mouse to sight an approaching ship in your cross-hairs. A target box will light up when an enemy ship is in your sights, giving the craft's closing speed and range. Click the mouse button to fire.
Whenever an enemy fighter gets past you, it will fire on your ship. You will see the screen flash and shake if they score a hit. Watch your shields - when they go, so do you.