What is the best way to hide a websites folder/ directory files

Create an index.html or index.php file inside, for example, the img folder; so when a visitor attempts to visit yourwebsite.com/img/ and see the content, index.html will be showed instead.

If your server is Apache you can setup an .htaccess file and build rules to hide/redirect and return a 404 Error page.

If your server is IIS then you can modify the "URL Rewrite" rules in your IIS Manager to return a 404 Error/Forbidden page.

See: .htaccess tips & tricks