With Finder active (click the desktop first), ⌘ + N opens a new window. In almost every other app ⌘ + N means “new document”.
Open a new Finder window
⌘ Cmd+N
Your computer or browser handles this shortcut before a web page can see it, so practice it on the keyboard below. On the real thing, just press the keys together.
Click the modifier keys first (they stay down), then click the final key.