There are 29 flights connecting Seoul and Beijing every week, with the journey taking 10 hrs 45 mins on average.
Or, if you're fairly flexible on dates, try to get a last minute-deal by regularly checking the Netflights website.
Beijing actually has one airports to choose from – but Beijing Capital Int. Airport (PEK) is the usually the cheapest option. If you need to fly into a specific airport, adjust your search to focus on those flights. The most popular airlines include China Southern Airlines & China Eastern Airlines.
But if you can't find one which is right for you, try looking at flights to other cities in China to check out some other options.