Żurek - Don't try to speak ( Polish language difficult language ), such a good soup. Bigos - Classic and the most popular in Polish food, cabbage, sausage and even more I recommend to watch below video, these guy from sweedish but speaking with problems in Polish And the last maybe Pierogi Ruskie? Ruskie - Russian, dunno why its called this but nevermind. Pie and stuffing, simple and tasty.
I know maybe in the pictures doesn't looks like sometimes good, but trust me its very good taste and one of them you should make
I don't know any polish fast foods, the most popular its ofc kebabs, hamburgers and pizzas nothing special i recommand traditional polish kitchen you will be not disappointed