Korean food isn't authentic without gochujang and doenjang, two vegetable pastes used in many soups, strews, marinades, and dips. Most Korean cooks make them at home, buy you should be able to find them in many Asian markets in glass or plastic jars.
Among the ingredients you'll need to cook authentic Korean food are sesame oil, sesame seeds, gochujang paste, doenjang paste, soy sauce, crushed red pepper, sticky Korean rice, seaweed (several kinds), glass noodles, and dried shiitake mushrooms.