Train tickets between Beijing and Shenyang

How do I arrange for Train tickets between Beijing and Shenyang I will be arriving on July 28 at Beijing international Airport 8.05 PM from South Korea. I don't think there's time to clear customs to make the last air flight so I would like to go by train. How would I go about doing this transportation from airport to train terminal? And arranging train tickets? Your advice would be greatly appreciated. Thank you