How can I turn off cellular data connection on my Windows Phone? I want to make sure that my phone is using my Wi-Fi connection.

If you want to turn off the cellular data connection on your Windows Phone, you can follow this tutorial:

1. On Start, flick left to the application list, tap "Settings > Cellular". You will see the Cellular screen showing up with these settings:

Cellular

Active network
   Vodafon

Data connection
   On 

Data roaming options
   roam 
      Depending on your service agreement, 
      you might pay more when using data roaming.

   don't roam
      When entering a roaming area, 
      your data will be turned off.

2. Tap on the "Data connection" value icon to turn it off.

Your Windows Phone can only access Internet with Wi-Fi connection now.

Keep the cellular "Data connection" off until when you really need to read emails or access the Web and there is no Wi-Fi available to save money on your cellular data cost.