Go to Settings > General > About and look for your device's IMEI
Before trying anything else, you can try simply dialing *#06# on your iPhone (or any mobile phone), this should retrieve the IMEI number.
The iPhone 5/5c/5s/6/6+ and the original iPhone have the IMEI number engraved on the back of the phone, near the bottom.
For find imie, 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)