If somebody dies in hospital, the doctor attending the patient will issue a Medical Certificate of Cause of Death. You will need this certificate when you register the death.
If the person has died at home, contact their doctor. The doctor will confirm the death and issue the certificate.
In the case of unexpected or sudden deaths, the emergency services should be contacted in the first instance. The death will be reported to a coroner, who may decide that a post-mortem is needed before a medical certificate can be issued.