It occurred to me, that it is difficult for some people to find where the SIM card slot is on the iPhone 3G. So I figured I would make this tutorial to help out.
The iPhone 3G comes with a tool that is designed specifically to remove the SIM card.
You can use this tool or a paper clip to simply poke the hole on the top of the iPhone 3G. This will eject the door for the SIM Card.
Now you can simply remove or insert the card into the tray, then shut the tray.