If you come to Kraków by plane, you should read this!
Recently Krakow-Balice Airport gained a new, fast train connection to the center. Railway station is located opposite to the terminal. Just follow the signs (see the plan attached). The ticket costs 8 złoty's. There are ticket machines on the terminal, on the platform and most likely on the train. Your final stop is Kraków Główny (Main Station).
You can also use a bus but it takes ages to get to the center.
Have a nice trip!