In order to be able to know how to unlock iPhone 3Gs you must have a TurboSim unlock card. You can go on line and search for ‘turbo sim’ to locate this card. You will spend as much as $10.00 for the equipment. When you purchase and receive this unit you need to look over your phone and find the slot that holds your SIM Card. It is probably at the top of the phone. To remove the SIM card you will need a paper clip to push in the little button that releases it. Put the new card you purchased inside the cradle and make sure it faces the same way as the card you removed.
Put your SIM card from your provider atop the TurboSIM card taking care to have the leads make contact with the conductors on the other card. You may have to cut the card to fit. Put both of these into your iPhone. If the phone says that you have no service you need to wait until the TurboSIM has completed its cycle. When the message disappears you will be able to use the phone normally to place calls, text, or surf the web.