Have you never used Joomla? You have some learing to do!
FTP to te server to see error files, location depends on how old the site is, you are lookig for error.php which could be in the site route or in logs directory or administrator/logs directory But if you are new to this you may not understand what they tell you.
Debug is turned on in VirtueMart Configuration/Shop under Advanced Settings (and also in Joomla Golgal configuration/System, but you may not need this).