Choose Operators APN Internet Setting for your iPhone 3G 3GS 4 and 4S , you can very easy to  install the APN Apps  Setting .

You must turn the Data Roaming , and 3G ON > go> Settings - General -Network

1. on your iphone Safari go to this web site by using the WiFi ( if you don't have the data connection set up yet)

go to  www.unlockit.co.nz       

2. click Continue button , then click Custom APN

3. Now in here you can choose your country , and Carrier - what provider you using on the iphone then click Create Profile

4. Now you can just click the install , after thatits done the APN Apps installed on your iphone, and your iphone internet setting its set ready to use your internet from your Cellular Data