You can use event/observer method of magento to do something after order is placed.
you can use this event sales_order_place_after
Just create one module to listen magento observer/event.
In your /app/code/local/{namespace}/{yourmodule}/etc/config.xml:
<config>
...
<frontend>
...
<events>
<sales_order_place_after>
<observers>
<unique_event_name>
<class>{{modulename}}/observer</class>
<method>your function name</method>
</unique_event_name>
</observers>
</sales_order_place_after>
</events>
...
</frontend>
...
</config>
And then create an Observer class at /app/code/local/{namespace}/{yourmodule}/Model/Observer.php
class <namespace>_<modulename>_Model_Observer
{
public function your function name(Varien_Event_Observer $obs)
{
whatever your logic put here
}
}