If you are talking about client or server SSL certificate then you need to import valid certificate file and its associated private key file. Once it is imported, you need to call both files under client or server SSL profile. Once it is configured, you need to call created SSL profile to Virtual Server.
In some cases, you also need chain of certificate or bundle certificate which is used for creating chain of trust between the CA that signed the certificate and the CA that is already trusted by the recipient of the certificate.
The step for configuration is given in below article,