Hi Tasi_68,
You will find two settings on the Lumia. One is Settings>WiFi which will control Wi-Fi settings. This setting will always override the Mobile network connection when a Wi-Fi network which is known is in range. If that happens the mobile data connection will not be used as long as the Wi-Fi is active.
Second is the Settings>Mobile Network options where you can switch off or on mobile data or make other adjustments.
You can also use the 'Network Setup' app from the Nokia Collection in Marketplace (if available). This app will setup settings for Internet and MMS using many available carrier presets which you can select or you can choose to do so manually.
Again, once a known Wi-Fi connection is available the Lumia will connect to that and not use the mobile data connection (which will remain active, but not in use).
Hope this helps!
