You can buy a Yang cheng Tong Metro Card at 7-Eleven which is at the arrival hall, A5 Gate. Its the only 7-Eleven at the Guangzhou Baiyun Airport. The card is 20 Yuan, but you need to adding money to your card.
You may buy a Yang Cheng Tong card(the card has money) at the Metro Customer Service Center but other services, like adding money to the card or getting a refund, are not available; please check with the Service Center if you have questions about your card.
If you want to refund the money of the Yang Cheng Card you have to go to the Tixilu Metro Station or the Gongyuan Qian Metro Station.