Every device has a unique 15 digit code called an IMEI number, like a fingerprint.
There are a few different ways you can find it:
By entering *#06# on your device's dialer
By checking your device settings
By looking on the back of your device, beneath the battery, or on the SIM tray
By asking the the company or network you first bought it from
By checking the original invoice or receipt from when you first bought it
By checking the original box, if you still have it.