Generally speaking, the key that serves a role similar to the Alt key on Windows is the Option (⌥) key. It's located right next to the Command (⌘) key, which replaces Ctrl on the Windows keyboard.

Open the Alt+Tab view.; Use the Tab key to navigate to the window you wish to close; keep the Alt key pressed all this time. When navigating to the window you wish to close, press the Ctrl+W keys ...

To use Windows Alt codes, simply press and hold the Alt key while typing a special character's numerical code on the numpad. And if your keyboard doesn't have a numpad, you can use the on-screen ...