Even though you can now use your SIM card with mobile data in entire Europe, you still may want to consider buying a Polish SIM card.
You need to register it, according to the law, but you can do it at the operator's shop - all you need for the registration is your national ID/passport and the SIM card. 

The prices of the Internet are rather similar for all mobile phone operators. Erasmus students usually choose Play, T-Mobile, Orange, Mobile Vikings, or Virgin Mobile.