The Data Usage app comes already installed and is used to monitor and view data usage and set data limits, notifications and restrictions.
From the Start screen, swipe left to display the Apps list.
Alternatively, tap All apps (located in the lower-right; may require scrolling).
Tap Network & wireless.
Tap Data usage.
From the 'Overview' screen, view the data usage info.
To view data usage by app, swipe left or right to view the usage screen.
Data usage info provided by the device may differ from actual usage.
For data usage info provided by Verizon: