For installing an SSL certificate for a domain the domain should have a dedicated IP. First you need to create CSR (certificate signing request) from the WHM for the domain you wish to install the certificate. When you generate CSR, you will get the key also. With the CSR, you need to contact SSL cert providers such as godaddy and buy the certificate from them.
With this cert and the key, you can install certificate for the domain from the WHM.