Below is an overview of how the crontab file format

Edit your crontab with:

crontab -e

To run crontab every 5 minutes add the following to your crontab file:

*/5 * * * * /path/example-script.sh

The same applies if you want to run the script directly from the crontab, see the example below:

*/5 * * * * grep "darth vader" /var/log/maillog | mail root

Source :
http://linuxmoz.com/crontab-every-5-minutes/