Summarize the following:
The iPhone 5, 5c, 5s, and the original iPhone have the IMEI number engraved on the back of the phone, near the bottom. If you need the MEID number, take the same number but ignore the last digit (IMEI is 15 digits, MEID is 14 digits).  GSM networks like AT&T and T-Mobile use IMEI numbers. CDMA networks such as Sprint, Verizon, and US Cellular use MEID numbers. If you are using an older iPhone, see the following steps. See this guide for details on removing the SIM card tray for your specific model. Your IMEI/MEID number is printed on the tray. If you are on a CDMA network (Verizon, Sprint, US Cellular), both codes will be printed as one. To determine the MEID number, ignore the last digit. This can be found on your iPhone's Home screen. This step works for any iPhone or cellular iPad. Tap "About" in the General menu. This will display your IMEI/MEID number. If you want to copy it to your iPhone's clipboard, press and hold the IMEI/MEID button in the About menu for a few seconds. A message will appear letting you know that the number has been copied. If your iPhone won't turn on, you can connect it to your computer and use iTunes to retrieve the IMEI/MEID number.  Plug your iPhone into your computer and open iTunes. Select your iPhone from the Device menu in the upper-right corner of iTunes and then click the Summary tab. Click the "Phone Number" entry next to the image of your iPhone. This will cycle through the ID numbers for your device. Copy the IMEI/MEID number. If both are displayed, check what carrier you use to see which number you need. GSM networks like AT&T and T-Mobile use IMEI numbers. CDMA networks such as Sprint, Verizon, and US Cellular use MEID numbers.

Summary:
Look at the back of your iPhone 5 or original iPhone. Check the SIM card tray of your iPhone 3G, 3GS, 4, or 4s. Open the Settings app. Tap General. Tap IMEI/MEID. Find the IMEI/MEID using iTunes.