What is a Virtual Machine
A virtual machine is a computer running inside another computer. You can run any Operating System using Virtual Box just download the ISO of the operating system and load it into your created Virtual Machine and start, it will then launch as a first time operating system. You can run multiple OS's at once E.G have windows as your main OS and run Linux and WindowsXP in virtual machine. What ever you do in the created OS will not have any effect on you main OS except storage will be consumed where it has been allocated. If you get a virus or malware on your Virtual Box OS it will have no affect on you computer unless it is transferred via internet. When performing attacks it is safer and more anonymous to use a virtual box OS because the hardware cant be traced to you but your internet actions can be if your not safe.
I have Linked Virtual Box below as well as some free Operating Systems to download.
Download Oracle Virtual Machine
Operating Systems - Free
Before downloading any of these research what they are and make sure they are the OS for you and make sure you install them properly failure to research and read how to install can lead to your PC without an OS. I have found that Linux Based OS's have been a lot easier to install and would recommend Ubuntu or Kali Linux for new users. All downloads below are links to the download website or a direct download link which you will be prompted to install. The file has to be an ISO file or DVD type file for Virtual box to launch it. I have also included images which are uick previews of the OS's appearance.