Supposing you are running Windows inside Virtual Box, what you have to do is:

• Create a folder that will be the shared folders between these two OS;
• Start Windows on Virtual Box;
• Click on "Devices" --> "Shared Folders" --> "Shared Folders Settings";
• Select "Machine Folders" --> click on the button with a sign of "+", then add the path of the folder you created before on Linux;
• Tick the "Make Permanent" checkbox;
• Go to "Network" --> Select "NAT" from the drop down options for "Attached to" ;
• Go to "File Explorer";
• Click on "Devices" --> "Insert Guest Addition CD Image";
• Run the installer you just have enabled and then Reboot your Virtual System.

At starting up Windows again:

• Go to "File Explorer";
• Click on "Tool" --> "Map Network Drive";
• Now you have to select a drive letter and the folder you want to as shared;
• A new window should appear and this is the place where you can drag and drop files, documents, music...
  

If you want to follow these processes through a video, please consider this tutorial: