How switch to composer installation?

I have installed Drupal manually few weeks ago. At the moment I want to switch to composer installation, so I will be able to update Drupal by composer. After backuping site, I try to follow:…

However still looks that something missing: My setup is local – nginx+raspbian on raspberry PI3. I already have installed composer&drush. At the moment drupal’s installation is in /usr/share/nginx/html, including composer.json. I checked composer.json and it looks ok with drupla8 repositories and installation’s directories. Now I need to know how go a head: Looks that need to run this command:

composer create-project drupal-composer/drupal-project:8.x-dev my_site_name_dir --stability dev --no-interaction

However, please advise if need to delete all files - core etc, before this command, or need to run this command on another directory? What dir should replace "my_site_name_dir" regarding my setup? How can I keep my current site's configuration?