What's an IMEI?
An IMEI number is a 15 digit code that helps mobile service providers identify your device. This is often used when seeing whether you can switch to another carrier's network.
Finding Your IMEI
Go to Settings > About. Your IMEI number should either be listed on that page, or inside a related section.
Go to Settings > General > About. Your IMEI should be towards the bottom of the page, underneath the "Primary" section.