In order to setup automatic content expiration you need to do the following:
1- Set the content expiry date.
2- Create "Scheduled Move" workflow action based on the "Expiry Date" to move content from Published to Expired stage. Add this action to your Published stage.
3- To guarantee the author will set the exp date, make it a required field in the Authoring Template. Also, set a default Workflow and a range of dates to be selected from. Click the icon besides "Expiry Date" under the AT "Default Content Settings" for options.