The iPhone is much more than a phone. More like a tiny computer, the iPhone can go anywhere you go and help you keep tabs on your life when you aren't near a computer. Using the iPhone, users have access to e-mail, Internet and more, with or without connecting to Wi-Fi. If no wireless networks are available, the iPhone will connect to the 3G network, a cellular data network, automatically. If it does not automatically connect to the 3G network, you can connect manually.
Tap the "Settings" icon on the home screen of your iPhone.
Tap "Wi-Fi" and then tap the "On/Off" button to turn Wi-Fi enabling off. The iPhone should automatically default to 3G, but if it does not, you can enable 3G manually.
Tap the "Settings" button at the top left of the screen to take you back to the Settings page.
Scroll down and tap the "General" icon.
Tap the "Network" icon and tap the "Enable 3G" button to turn 3G on.
Check the top left corner of your screen to see if "3G" is displayed next to the signal strength.