Oracle's VirtualBox virtual machine manager program runs guest operating systems inside the operating system on your computer. Each guest operating system has its own virtual hardware and normally runs in a window on your desktop. You can also use a virtual machine in full-screen mode, covering up the rest of your desktop. You can switch to full-screen mode using a menu option or a keyboard shortcut, but you can only exit full-screen mode with the keyboard shortcut.
Start a VirtualBox virtual machine by double-clicking its name in the VirtualBox window.
Click the "Machine" menu and click "Switch to Fullscreen" or press the "Ctrl" key at the right side of your keyboard and press "F" at the same time.
Click the "Switch" button in the confirmation window that appears.