Okay, previous post was meant as a joke, there is the "Flight Mode" - if you enable it all antennas will be powered off and no data can be sent/ received which will protect you from using up your data plan.
I guess that you have a 3G model which is actually connected to cellphone network.
Same as on an iPhone you can disable in the "general" settings, "Network" the Mobile Data - this way no data will be sent/ received using your data plan, however when you are connected to a WiFi network the device will send and receive. In case you want to connect while "on the run" you just enable the mobile data again.
In case you have no Wifi at home/ around it is recommended to have an unlimited Data plan or one which just reduced the speed once the included data amount is consumed.
Hope this helps.