Look at the back of your iPhone 5 or original iPhone. The iPhone 5, 5c, 5s, and the original iPhone have the IMEI number engraved on the back of the phone, near the bottom.
Check the SIM card tray of your iPhone 3G, 3GS, 4, or 4s. See this guide for details on removing the SIM card tray for your specific model.
Dial the IMEI code. You can retrieve the IMEI/MEID number on virtually any phone by dialing in the universal code. Dial *#06#. You typically will not need to press the Call or Send button, as the IMEI/MEID number will appear as soon as you finish dialing the code.
Copy the number. Your IMEI/MEID number will appear in a new window on your phone. Write the number down as it is not possible to copy and paste it from your phone's display.