in reply to cronjob to run CGI every 30 minutes?

Will the script be executing on an Unix machine? If so, check out 'man cron' and 'man crontab'. If you don't have access to the man pages, you can read them here:

man cron
man crontab

Maybe someone else here can provide you with an answer if the script will be running on a Windows machine, or some other OS.