How to activate cell phone in order to use it in other country?

I wane activate my cell phone because I wane use it in Poland when I'm go on vacation. I wane buy sim card in there, and I know that I have to unlock something in order for the sim card in Poland to work, but where I have to go. I wane do it by my self . I don't wane pay for this activation, because I wane lear it by my self.