All most travellers need to rent a car in PL is 18 years of age, a credit card (not debit), and a valid foreign driver's licence. Be aware, however, that those from countries that didn't ratify the Vienna Convention on Road Traffic (United States, China, Australia...) cannot legally drive on their home licences; technically an International Driver's License is required in those cases (in the US these are issued by AAA for a fee). If you don't have a license in line with the Vienna Convention, or the required IDL, you may be denied the ability to drive a rental car. Though some car rental companies (the dodgier ones) will still rent you a car (since it's good for their business to ignore international law), be aware that you are assuming full liability if you get behind the wheel; not only can you get a citation from the police, but if an accident were to occur, you would be fully responsible for any damages, regardless of the circumstances.

If you're looking to leave the country in your rental car, be aware that you can't cross the borders into Ukraine, Belarus, or Lithuania in a rental car.