To change your foreign currencies into złotys( PLZ) in Poland, it is best to go to a KANTOR (Change) and compare their different rates. Shop around, the rates can vary a lot.

The traditional banks'  fees are higher. If you are an expat or want to buy an apartment it is necessary to open a bank account .

Polish banks are very bureaucratic but the most foreigner-friendly bank seems to be BZWK Zachodni Bank which has several branches all over the country (it belongs to an Irish Bank ). One can open an account in Euros, Dollars or Złotys.

Also using your credit/debit card at the cashpoint (Bankomat) will get you a good exchange rate.  Be aware that many banks charge for this service.  Don't worry the cashpoint  will be in English once it has read you card so they are easy to use. 

In all of Poland shops are unimpressed to be presented with large notes so be sure to get small notes from the travel exchange.