Just like domain names you can buy SSL Certificates from third party websites or you can buy it from here. First you have to generate CSR (Certificate Signing Request) here with your business details. If you are unsure about what to fill in CSR, contact your certificate vendor, since they will be the one verifying your business.
From your certificate vendor you will receive three things, a certificate, the RSA key and a CA bundle.
Once you get hold of the CSR, go to your primary domain and click SSL Certificate and then import the certificate you downloaded from your certificate vendor.