Below is the planetoids game that you will ultimately program in Lab 1. The purpose of this "introduction" is to emphasize that the coordinate system is centered in the top left of the screen. Therefore the origin x = 0, y = 0 is in the top left of the screen. It is also important to appreciate that y increases towards the bottom of the screen. This is why +y points down in the planetoids game below and +x points to the right like normal.
This is reminiscent of falling object problems where downward motion is positive. Besides that everything is the same.