Is there a way of getting a notification when you are using data?
There's no way in iOS that would send a notification if you start using mobile data rather than Wi-Fi.
However, you can either disable mobile-data entirely; disable Wi-Fi assist, which is a feature baked into iOS that lets the phone switch automatically to mobile data if the Wi-Fi signal is weak; or disable mobile data on a per app basis.
If you are concerned about a data hungry app such as video streaming or social networking apps, you can selectively disable mobile data for them by going to Settings app → Mobile Data and turn off the switch shown corresponding to the desired apps.