In the majority of cases a visa is not required to enter Poland. Poland is part of the Schengen Area – to enter the zone, citizens of USA, Canada or Australia only need a passport valid for six months beyond planned departure date. If you are not sure about the rules applying to your country, take a look at the list of visa-free countries on the Ministry of Foreign affairs website:
http://www.msz.gov.pl/en/travel_to_poland/entering_poland/visa_free/