The 2D coordinate system that we use in video games is a relatively straightforward one. Everything is measured in pixels from the top left corner. Nearly all of the drawing commands that we do will be expressed in these coordinates.
The top left corner, (0,0) origin with width and height has a value of zero, zero. The x-axis value (positive) increase moving the left to the right direction. The y-axis value increase moving top to bottom direction.