I'm settin my new iPhone up and it says no service. How do I fully activate it?

This is a loaded question. First, your carrier should be able to help you out with activating your service. Contact them first.

You might be able to set up the phone over Wi-Fi or when connected to your computer running iTunes.

i have the same problem, i've tried running it on itunes still doesn't work, itunes says "Only compatible SIM cards from a supported Carrier may be used to activate iPhone. Please insert the SIM card that came with your iPhone or visit a supported Carrier's store to receive a replacement SIM card." My iphone came from other country,, and carriers here doesn't support iphone 3gs,,, i can't even use it as an ipod rather... help us please....