There are at lease SIX different ways to find your 15 digit IMEI number, depending on your phone model:
1.Go to the Phone app and type *#06# click send.
2.Go to Settings/General/About
3.At the back of the iPhone, in the small writing at the bottom.
4.Connect your phone to your computer, open iTunes, you will find it there.
5.On the setup screen, look for an 'i' which has the IMEI.
6.Take out the Sim tray, it has the IMEI number.
(avoid this method as the tray could have been swapped with a different one)