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 5
Slide two fingers downwards starting from the top of the screen.
Step 2 of 5
Press the network icon.
Step 3 of 5
Press the indicator next to the mobile data icon to turn the function on or off.