From MIM Wiki
There are two steps that you need to take to stay in Poland over 3 month legally. The whole procedure is relatively simple. All you need to do is get a residence permit for a fixed period of time and then register your place of residence. Below we walk you through how it is done.
 Step One - register your stay in Poland:
You have to go to the office of Mazovia Voivodship Office (Mazowiecki Urząd Wojewódzki) on Długa 5 street in Warsaw within your first 3 months in Poland.
Make sure you have the following legal documents with you:
- Passport or other form of ID with a photo that could be taken as a proof that you are who you claim to be.
- A document giving a reason of your stay here, for instance:
- possibility of future employment for example - statement from your future employer plus proof of health insurance and proof of having enough money to live in Poland
- intention of studying in Poland: confirmation of admission to university plus proof of health insurance and having enough money to live in Poland
- being married to a Polish citizen - act of marriage
The most important is you have to go to the legal office yourself and no-one else can do it for you.
More informations can be found here.
 Step Two - register your place of residence:
In Poland all citizens are obliged to indicate their place of residence and this rule as well applies to foreigners staying in Poland for over 3 months. So the next stage of your registration is letting the local district public office (Urzad Gminy) know what is your place of residence.
For a list of local district public offices' address, check here.
There are two options:
- temporary registration
- permanent registration
The temporary registration of your stay lasts between 3 months and 5 years whilst the permanent is for over 5 years.
To register your place of residence you need to take some documents to the local office. You need:
- Residence permit
- ID or Passport
- Lease (if you are choosing temporary registration) or act of ownership of the flat (if you are choosing permanent registration)
- Filled out form.