use the sim ejector tool that came with your iPhone or a thin paperclip end/tip and stick it into the small hole on the side of the iPhone. This will pop the sim tray out and you can insert your sim through there. If your iPhone is above the 3GS model, then the sim tray is on the side of your iPhone. If you iPhone is 3G or 3GS, it is on the top. Make your you don't poke the microphone. You will know it's not a microphone because there's a sim tray outline which is the sim tray. the hole that you use to eject the sim is within that sim tray outline.
Hope that helps