I inserted a new SIM card into my phone and am asked for a PIN. How can I unlock it?

Network Lock

If your device locks when used with a SIM card from a region authorised for that device then it strongly suggests that device has been locked by a mobile network to only work with that network's SIM cards. 

Samsung does not lock any of its devices to any particular network. You will need to contact the original service provider or retailer for your handset to request an unlock code. This is not something that Samsung can do on your behalf. Some service providers may allow you to unlock the device for a small charge or for free, whereas others may not allow the device to be unlocked at all.