Drupal 8 needs Drush version >= 8. Drush 8 has the dependency of Composer. If you don't have composer download and install it using the link https://getcomposer.org/Composer-Setup.exe for windows. Download and install Git. Open your Command Line or GIT’s bash and try the following commands,
composer global require drush/drush:8.*
This command will change the current directory to “C:/Users/User/AppData/Roaming/Composer” and starts installing composer. If you have git bash, you can type the following command to find out the installation path.
To check the working of the Drush use “drush version” in your cmd. This will return the version of the Drush installed in your machine.