Move the curosor on screen to aim your Warthog's gun turret, and hold down the left mouse button to shoot. Move your Warthog around by using the W A S and D keys on your keyboard. Down on the lower portion of the screen, you at the far left have the speedometer - this shows how fast your Warthog is moving. Next to that, is your ammo meter - press the space bar to reload your chain gun as it gets low. Next to the ammo display is the damage indicator, showing how much damage has been currently done to your vehicle. Just next to that and up slightly is a number - this is how many extra lives you currently have left. Off to the right of this, pretty much in the center is your current mission objective status.