Raise the alarm as soon as it is safe to do so, perhaps by activating an alarm during the robbery. Make sure you call the police before you call anyone else. Close the premises to the public and keep out unauthorised people. Isolate the area for later forensic examination, in particular fingerprints. Get staff to independently note down a description of the offender and the words used in the crime.
First impressions are vital.
Do not make statements to the media without clearing it with the police. Do not comment on how much money was involved except to the police. Give police all details, even those which seem insignificant to you. However, the result will only be as good as the description provided. Use a reference point in the store to determine the offender's height, for example, some shelves or mark on the wall.