Cron jobs are set up as a server function.
If you have a control panel to your server, that would be the easiest way to create the cron job. If you are on a shared server, you may have to ask your provider to create the job for you.
If you do it yourself, you'll need to provide the ABSOLUTE PATH to the cron script.
to find it:
<? echo $_SERVER["DOCUMENT_ROOT"] ; ?>