How to insert or remove the SIM card on the iPhone 3G or 3Gs.
The SIM card on the iPhone 3G / 3GS is located on the top of the device, use a paper clip or the SIM extraction tool (which you have probably lost!) to pop out the SIM card slot by inserting it into the hole in the top of the iPhone.
Next slide out the SIM from the holder, check the diagram below for easier instruction.
Inserting the SIM card is just a case of placing it in the SIM holder and carefully inserting it into the iPhone.