Put your iPhone into recovery mode. If you do not know how to do this then you can follow this tutorial.
Click to select About This Mac from the Apple menu at the top left of your screen.
Click the More Info... button from the window that appears.
System Profiler will open. Select USB from the Hardware menu at the left.
Select Apple Mobile Device (Recovery Mode from the USB Device tree in the top right panel.
Copy (highlight and press Command+c) your ECID from the Serial Number field.