My thoughts on learning vocabulary in general: https://www.italki.com/discussion/91279
The best words to learn are the ones you will personally see, hear, and use the most often^^
There's talktomeinkorean.com, howtostudykorean.com, quizlet.com, memrise.com, ankisrs.net, and many others. Too many to remember.
Here's a dictionary: endic.naver.com
Remember, anything can be a source of vocabulary:
Try writing or saying something. Think of a word you don't know? Look it up, use it, learn it.
Try reading or listening to something. See/hear a word you don't know? Look it up, use it, learn it.
Of course, try to pick materials at an appropriate level/personally relevant and prioritize learning words (conversational vocabulary over vocabulary to read newspapers).