To change your foreign currencies into złotys (PLN) in Poland, it is best to go to a KANTOR (bureau de change, currency exchange office)) and compare their different rates. Shop around, the rates can vary a lot. Words "skup" or "kupno" mean "buy rates" and "sprzedaż" means "sell", so if you want to check how much zlotys you will get for euros or dollars, you need to look at "skup/kupno". The traditional banks' fees are higher.
If you are an expat and want to buy an apartment it is necessary to open a bank account. One can open an account in Euros, Dollars or Złotys.
Also using your credit/debit card at the cashpoint/ATM (Bankomat) will give you a good exchange rate. Be aware that many banks charge for this service. ATMs offer service in English so you won't have any problems with withdrawing money.
Try to avoid changing money at the airport on arrival as the rates are not favorable. If you do not have an ATM card, then change the minimum to get you into town and then change the rest there. If you do have an ATM card, there are ATMs in the arrival hall (practically on every airport in Poland). There are at least two of them so if one does not work, try another (they are usually not located together).
Before leaving for your Poland trip, check with your bank to see if it is affiliated with any banks in Poland and go to those bank ATMs to withdraw money. That will save you any handling fees.