Poland is a member of a Schengen so nationals from EU need only a valid national identity card or passport for entry.
Nationals of non-EU/EFTA countries generally need a passport for entry to Poland and most of them need a visa.
Only the nationals of the following non-EU/EFTA countries do not need a visa for a trip to: Albania(1), Andorra, Antigua and Barbuda, Argentina, Australia, Bahamas, Barbados, Bosnia and Herzegovina(1), Brazil, Brunei, Canada, Chile, Costa Rica, Croatia, El Salvador, Guatemala, Honduras, Israel, Japan, Macedonia(1), Malaysia, Mauritius, Mexico, Monaco, Montenegro(1), New Zealand, Nicaragua, Panama, Paraguay, Saint Kitts and Nevis, San Marino, Serbia(1)/(2), Seychelles, Singapore, South Korea, Taiwan(3) (Republic of China), United States, Uruguay, Vatican City, Venezuela, additionally persons holding British National (Overseas), Hong Kong SAR or Macau SAR passports.
(1) the visa is needed unless you have a biometric passport,
(2) If you have a passport issued by the Serbian Coordination Directorate (for residents of Kosovo with Serbian passports) you do need a visa
(3) The visa is needed unless you have your ID number stipulated in your passport.
The legal currency in Poland is Polish Zloty (PLN) which is devided into 100 grosze. 1 PLN corresponds to approximately 1/4 EUR.
In the most of shops you can pay with cards among which Visa, Visa Electron, MasterCard and Maestro are commonly accepted. Other cards, eg. mEx or Diners' Club are not popular and it is better not to rely on them.
Private currency exchange offices (called kantor) are quite common. You should be awere that those located in 'tourist' areas often offer bad rates. "Interchange" kantor locations (which can be recognised by their orange colour) give really bad rates.
You can also exchange money in a ATM machine, but you must be awere than your bank often charges you with big 'operational costs' then.
You can find a pretty good info at wiki page: