Magento default have a cron.php file located in the root directory. To set a cron in magento you have to create your own module first, and in the config.xml you have to declare the method which will fired and the time. Below is a dump of a etc/config.xml file of a custom cron module

    <?xml version=“1.0??>
    <config>
    <modules>
    <Wl_Cronset>
    <version>0.1.0</version>
    </Wl_Cronset>
    </modules>
    <crontab>
    <jobs>
    <Wl_Cronset>
    <schedule>
    <cron_expr>01 00 * * *</cron_expr>
    </schedule>
    <run>
    <model>cronset/expired::productExpired</model>
    </run>
    </Wl_Cronset>
    </jobs>
    </crontab>
    <global>
    <models>
    <cronset>
    <class>Wl_Cronset_Model</class>
    </cronset>
    </models>
    </global>
    </config>

As per mentioned in this xml your cron will be fired every day @ night 00:01 min

Now you have to write your own functionality in your module Model file, Here my model file name is Expired and the function which will trigger is productExpired().

    <?php
    class Wl_Cronset_Model_Expired extends Mage_Core_Model_Abstract
    {
    public function productExpired()
    {
    // Your code goes here
    }
    }
    ?>

Now login to your server to call the magento root cron.php and set to be fired in every minute.