Download the Slackbasics book
And read the chapter 21.1. Scheduling tasks with cron
Of course you may search with Google. I just like Slackbasics and Slackbook
. Good resources to learn Linux.
To do some job every 15 minutes, you have to edit the crontab for a certain user ('crontab -e') and then add there something like:
*/15 * * * * /bin/bash /path/to/script.sh
Note that editing crontab is in Vi-style, so you need to know the basic Vi