You can limit your data usage by turning off mobile data. You'll then not be able to access the internet using the mobile network. You can still use Wi-Fi even though mobile data is turned off.
Slide your finger downwards starting from the top of the screen.
Press Mobile data to turn the function on or off.
Press the settings icon.
Press Wireless & networks.
Press Mobile data.
Press Networked apps.
Press the field next to the application name in the "Mobile data" column to turn the function on or off.
Press the Home key to return to the home screen.