When you're connected to Wi-Fi, your iPhone won't use data. Therefore, make sure you connect to Wi-Fi whenever it's available!
How Do I Use Wi-Fi Whenever Possible?
- Open the Settings app.
- Tap Wi-Fi.
- Make sure the switch next to Wi-Fi is turned on.
- If you're in a new area, connect to a nearby network by tapping on it under Choose A Network...
- Once your iPhone connects to a Wi-Fi network for the first time, it will remember that network and how to connect to it!