The Polish currency is PLN (złoty). 1 PLN is around €0.24 / $0.30. ATMs/cash machines (“bankomat” in Polish) are plentiful in Warsaw, so you don’t need to carry much cash with you. You can pay with all the popular debit or credit cards in most shops, but there are some places where either only cash is accepted or it is possible to pay with a card only if you spend more than 10 or 20 PLN. This practice is a legal grey area, sometimes seen in smaller shops.
You can also exchange some money in a bank or money exchange (“kantor” in Polish) – you will find plenty of both in the city center.
Note that some tourist-oriented places might accept EUR, but usually at a horrible rate (1 PLN = 1 EUR is not uncommon) – stick with PLN.
We use Type E power plugs in Poland, so remember to bring an adapter if you use a different standard.
Safe to drink, enjoy!
Polish is a very tricky language, and learning it just for this trip might not be the most efficient use of your time. Fear not, though, as younger people generally know at least some English. There might be a problem with older (50+) folks, though. You could try Russian or German, if you know them.
In any case, improvised sign language is universal.
Mind your belongings. There are pickpockets in Warsaw, so do be careful.