To install Composer on Windows you have to follow documentation https://getcomposer.org/doc/00-intro.md#installation-windows.
The openssl extension is missing issue
During installation "The openssl extension is missing..." issues could happen.
Installation tool helps you to identify php.ini file location. As you already understood, we have to do some modifications in php.ini file to fix the issue. If you have empty php.ini file, then just add following code there:
extension_dir = "ext" extension=php_openssl.dll
Otherwise open php.ini and uncomment php_openssl.dll extension and specify correct directory for PHP extensions.