You need a tourist visa to Beijing, China, you can apply for it from China embassy in Seoul or China consulate in Busan, but you should submit visa application to the travel agencies designated by China embassy in Seoul or China consulate in Busan. Please bear in mind that the Korean Alien Registration Card valid for 6 months is required. You can refer to Beijing Tour Packages(https://www.travelchinaguide.com/package/beijing.htm) to arrange your trip.