How to Program a Simple Space Shooter Game

When the green flag is clicked enemies (in my game basketballs) randomly fall from the sky. A variation you could easily add would be enemy ships running patterns in random order. You control a base ship that moves back and forth with the arrow keys on the bottom and uses the space bar to fire a laser. If the laser hits the enemies you score a point. If the enemies fall on you before you shoot them you loose a life. If you loose three lives the game is over. There are sounds when you fire your laser or are hit by an enemy ship.