In your home directory on the server create a folder with a meaningful name eg certRequests. Within the new folder run the following command to generate the CSR and private key. Ensure to update changeme to your domain name.
sudo openssl req -new -newkey rsa:2048 -nodes -keyout changeme.com.key -out changeme.com.csr
Two files are created in the directory. The *.csr file or it's contents are what to submit to the certificate authority of choice