You'll need to insert your SIM card into your iPhone before you can use it:
- Press the SIM eject tool (this comes in the box with your iPhone) or one end of a small paper clip into the hole of the SIM tray on your iPhone. Press firmly until the tray pops out enough for you to pull it out.
Your SIM tray's location will depend on your iPhone:
- if you have an iPhone 4 or 4S, the SIM tray is on the right hand side
- if you have an iPhone 3GS or an earlier model, the SIM tray is on the top
- Pull the tray out and place the SIM into the tray. Make sure the angled corner is at the top left of the tray
- Push the tray back into the iPhone
You can find out more about inserting your SIM here.