If you want to use only wi-fi for watching programmes on the app, make sure your phone or tablet is connected to a wi-fi signal and your own mobile data is switched off.
The way to do this will vary depending on your device, so if you need more help with this, check its instruction manual or go to the manufacturer's website and search for instructions on there.
Watching TV shows or films uses around 1GB of internet or mobile data per hour for SD and up to around 3GB per hour for HD.