I'm new to Linux. How can I copy a file or directory from the Debian OS to Mac or Windows?
You can share filesystems across these platforms but this requires some basic network/sharing configuration on all involved systems.
Sharing via Filesystems
On OS X you can enable File Sharing in System Preferences -> Sharing
By default only AFP sharing is enabled which may not be available in a standard Linux setup (and definitively is not available on Windows). To also enable Samba/Windows sharing, click on "Options..." and enable Samba sharing there.
Sharing via Syncing solution