if you use the Eclipse IDE, go to the market, serach for 'Liferay', install the plugin.
You'll also need an SDK which can be downloaded from Liferay.com
The Liferay CE source helps too.
In the process you'll configure Eclipse so that it knows about the SDK and the liferay runtime, you'll need to configure the SDK so that it knows where the Liferay Runtime lives, and once that is done you can create a new Portlet using the wizard which will deploy and run out of the box.
subject: Starting portal/portlet devlopment using Liferay 6