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 two fingers downwards starting from the top of the screen.
Press the settings icon.
Press Network and Internet.
Press the indicator next to "Mobile data" to turn the function on or off.
Slide your finger upwards starting from the bottom of the screen to return to the home screen.