Let's Encrypt is a Certificate Authority that lets you acquire an SSL certificate. For systems with shell access, it is recommended to install Certbot, an application that uses the ACME protocol. Certbot is packaged for many common operating systems and web servers.
sudo dnf install certbot python2-certbot-apache
If you run Debian Stretch or Debian Sid, you can install certbot packages.
When you initially run the command to obtain the certificate, certbot will create an account asking you for an email and agreement to the Let's Encrypt subscriber policy. You can automate that task by adding the --email & --agree-to primers