Do you have a Windows machine or a Linux machine?

Under Windows cron is called 'Scheduled Tasks'. It's located in the Control Panel. You can set several scripts to run at specified times in the control panel. Use the wizard to define the scheduled times. Be sure that PHP is callable in your PATH.

Under Linux you can create a crontab for your current user by typing:

crontab -e [username]

If this command fails, it's likely that cron is not installed. If you use a Debian based system (Debian, Ubuntu), try the following commands first:

sudo apt-get update
sudo apt-get install cron

If the command runs properly, a text editor will appear. Now you can add command lines to the crontab file. To run something every five minutes:

*/5 * * * *  /home/user/test.pl

The syntax is basically this:

.---------------- minute (0 - 59) 
|  .------------- hour (0 - 23)
|  |  .---------- day of month (1 - 31)
|  |  |  .------- month (1 - 12) OR jan,feb,mar,apr ... 
|  |  |  |  .---- day of week (0 - 6) (Sunday=0 or 7)  OR sun,mon,tue,wed,thu,fri,sat 
|  |  |  |  |
*  *  *  *  *  command to be executed

Read more about it on the following pages: Wikipedia: crontab

Install cron on Linux/Unix:

apt-get install cron

Use cron on Linux/Unix

crontab -e

See the canonical answer about cron for more details:

Installing Crontab on Ubuntu

sudo apt-get update

We download the crontab file to the root

wget 

Unzip the file crontab-0.20.tar.gz

tar xvfz crontab-0.20.tar.gz

Login to a folder crontab-0.20

cd crontab-0.20*

Installation order

python setup.py install

See also here:..

On CentOS/RHEL:

yum install cronie

Cron is so named "deamon" (same as service under Win).

Most likely cron is already installed on your system (if it is a Linux/Unix system).

Look here:

or there

for more details.