I took the list of Korean slang, found here: http://koreanlanguagenotes.blogspot.com/2005/11/do-you-know-any-good-korean-slang.html
When I was learning this, I thought that some wouldn't be very useful, or common. I was very wrong, since I live in Korea and hang out with almost only Korean people, I would hear almost every new phrase within a week of learning it. Some would take longer. Good luck!!