First make sure your Guest OS is turned off else you won't be able to make the changes.
Then on your VirtulaBox Management Page make the following changes as shown in the above image or continue to read below:
- Click on Settings
- Click on Shared Folders
- In 'Folder Path' select the desired Drive/Folder/File
- If you just want to read the files from your Guest OS the select Read-only else if you want to read/write leave it as it is.
- Give a name to your folder in 'Folder Name'.
- Select Auto-mount if you would like to mount your drive/folder/file on startup.
- Finally Click OK and that's all there is to 'Sharing Folders' in Virtual Box.