When applying SSL cert, very common to have generate some public key and private key for signing.
Here to generate a private key first
$openssl genrsa -des3 -out subdomain.domainname.com.key 1024
Then, generate a certificate request file (csr) using the private key you have generated on previous step.
$openssl req -new -key subdomain.domainname.com.key -out subdomain.domainname.com.csr
You will be asked to enter some information. The last two extra option can be left blank.
Then just submit your CSR file to your SSL cert.