I ordered my iPhone through AT&T. How do I determine which model I have?

There are several ways to check and see what model iPhone you own:

Option 1: On the Home Screen, click on Settings. Click General and then About. The "About" page displays your iPhone's hardware version, its serial number, its capacity, and its model number.

Option 2: With the device shut down, plug in the USB sync cable to an active USB port (or a powered USB hub connected to an active USB port). Press and hold the Home button, then power on the device by pressing the power button. Continue holding the Home Button until you see "Connect to iTunes" displayed on-screen.

Option 3: You can determine your iPhone model one of two ways: with Version 4.0 or higher of the iTunes software, click on "Check for Update" in the Summary pane. Or, if you are running an older version, connect your iPhone to that computer and launch iTunes. If your device appears in the Devices list with a yellow exclamation mark next to it, right-click (or Control-click) on the icon and select "Get info". The "Summary" tab in the resulting window reports your iPhone model.