How do I close a window on a Mac

If by "window" you mean a "desktop space," then here's your answer.

• four fingers up on the track pad (or somehow get tot he screen below)
• hover mouse over the one your want to close
• click the X button

If that doesn't work, go to the window/desktop space you want to close and make it so that it's not full screen anymore. Every time you make something full screen, it creates a new desktop space/window.