For find imei, you need remove sim card tray from device, and then you can find imei on it. (not on all models you can find imei via sim tray)

Go to Settings > General > About and look for your device IMEI.

Before trying anything else, you can try simply dialing *#06# on your iPhone (or any mobile phone), this should retrieve the IMEI number. Give it a moment and the IMEI will appear on the screen this little number dial trick actually works to retrieve the IMEI on other cell phones too, not just the iPhone