Great question! Managing your data usage is definitely important. You can view what apps are using data by going to Settings and then Mobile data on your phone. There is will show you the apps using data.
If you want to avoid the data usage on the apps you want to make sure you close out the apps when not in use. You can also make sure that they are not automatically updating whenever there is a new update to the app. To do this simply go to the Play Store and select Menu. Then go to Settings and there you will see the option to change the auto update to update over wifi only.
If you want to complete turn off data, just go to Settings and then mobile network and turn it off. Keep me posted if you need further assistance.
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