If your network service provider does not charge you a fixed fee for data transfer, there are a few things you can do to reduce your data usage (also presented on this video (English) from Nokia Care).
Change mail sync settings
If your phone syncs mail frequently, it uses more data. You can choose how often your phone automatically syncs mail.
If you have installed apps, such as news or weather apps, they may use data by syncing content. Make sure to check the sync settings and feedback sending options of the apps you have installed. On the start screen, swipe left, tap Settings > from settings swipe left to Applications tab, and check the settings of the apps.
Check data roaming settings
Connecting to the internet when roaming, especially when abroad, can raise data costs substantially. Data roaming means using your phone to receive data over networks that your network service provider does not own or operate.
To make sure you are not using your phone to roam, on the start screen, swipe left, tap Settings > Mobile network (cellular), and switch Data roaming options to Don’t roam.
Switch your mobile data connection off
If you want to make sure you’re not using data at all, you can switch your data connection off altogether.
Tap Settings > Mobile network (cellular), and switch Data connection to Off.