If you are physically located in Poland and do business in Poland, normal tax brackets apply (regardless of your customers' locations). I guess the simplest way would be to add your total income during a tax year, take the number to your accountant and he would calculate total tax amount owed. But that may not be enough because you need some kind of book-keeping, ie. each and every transaction must be available to tax office if needed. But Paypal automates most of the things so it's not time consuming. Or you could calculate it yourself (not recommended because you could miss some business expenses that could be written off from your total earnings amount).
