To be able to make calls, send SMS etc while you are abraod you need to select a network, from those available, in the country that you are in.
You can do this one of two ways:
Turn on the telephone when you arrive in the country that you are visiting. Introduce your PIN and leave your mobile to find and connect to a network. Once you are connected to a network you can use your telephone normally.
If more than one network exists in a country you are visiting you can select the one you prefer from the menu option in the telephone. This way you can select a network according to its services or its coverage. The roaming tariffs may vary depending upon which network you choose. For more information on how to make a manual network selection, please consult the user manual that came with the phone when it was purchased or ask Google.