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.
Step 1 of 6
Turn mobile data on or off
Slide your finger downwards starting from the top of the screen.
Step 2 of 6
Turn mobile data on or off
Tap the settings icon.
Step 3 of 6
Turn mobile data on or off
Tap Networks.
Step 4 of 6
Turn mobile data on or off
Tap the indicator next to "Mobile data" to turn the function on or off.