The Certificate Signing Request (CSR) contains your certificate application information and can be generated with your server software. Please contact your server administrator to obtain it or you can read the following article to generate it yourself. https://www.eurodns.com/ssl-certificate-faq/#howtogeneratecsr
IMPORTANT: Please note that if you need an SSL certificate for both: the host www and the APEX domain (e.g. www.example.com and example.com), the CSR must be generated for the common name www.example.com. If you generate the CSR for only the APEX domain: e.g. example.com, the SSL certificate will only be valid for the APEX domain (e.g. example.com) but NOT for the host www (e.g. www.example.com).
IMPORTANT: when you generate the CSR, a private key will be generated. Please keep this private key secret to avoid any other party misusing your SSL certificate.