Please note that in some cases it is required to obtain a VISA before entering Poland.
If you are a citizen of an EU member state the only thing you need to have to enter Poland
is a valid travel document or a document confirming identity and citizenship (i.e. your national ID).
The two main documents you will need to enter Poland if are not a EU citizen are:
a valid travel document (passport);
a visa (if required). Generally, you need a visa to enter Poland. There are, however, a number of countries that are free of this requirement. You can check if your country is on the list here: http://www.msz.gov.pl/en/travel_to_poland/entering_poland/visa_free/visa_free_countries.
Poland is a part of the Schengen zone, so you can apply for a Polish visa and travel all around Europe (excluding the United Kingdom, Ireland, Cyprus, Bulgaria, Romania, Croatia and states outside of the EU).