Public directory aren't really the realm of ExpressionEngine, that's a basic website question, which maybe more appropriate for Stackoverflow. That said, the general conditions for making a directory public is to make it world writable using permissions. World writable is consider
Generally, your ISP will provide you some access to change permissions of files and directories within your public_html folder. Most FTP applications also provide the opportunity to visually change permissions, so that you don't need to know the unix
chmod command. Obviously, there can be security implications for making a folder world readable, generally you don't want to allow executable files in those directors, more like images, pdfs, etc.