I will go to the center of Warsaw with Modlin Bus and then go to Kielce with a polish, it's the train station to far away from the city center?
Have you booked your bus ticket online? Depending on time of day you might find the bus is full and have to wait for another. If you have to change currency, don't do it at the airport as the exchange rate will be criminal. The bus will drop you in central Warsaw outside the Palace of Culture on Marszałkowska. There are a couple of Kantors for changing money in the underpass beneath the big roundabout to your right, one is a Western Union if I remember rightly. If you take a right turn at the roundabout, then you wil come to the main train station, Warsawa Centralna. Have a look on google maps, but it is very close by.
You're not really making yourself clear though, are you asking if the train station in Warsaw or Kielce is far away from the centre?