You will need your CSR (Certificate Signing Request) and certain required documents depending on the certificate you purchase. You can get your CSR from your hosting provider or your system administrator. Once, you purchase the certificate and process your order. You will then need to send your necessary documents to the respective company. For Domain Validated Certificates you will receive an Approval/ DCV (Domain Control Validation) email to prove you are the domain owner. For OV & EV certificates, you will need to provide the documentation requested.
It’s also important for you to know that in order to purchase an SSL certificate you need a Dedicated IP address which you can from your hosting provider.