To look up your International Mobile Station Equipment Identity number, dial *#06# on most phones. Your device screen should display the number, even before you click on the call button. However, this number may be different on some phones.
Alternatively, look for the IMEI number on the back panel of your phone. To do this, remove the back case of your phone, and then remove the battery. You can see the IMEI number printed along with other manufacturer related details. One more option is to look up this number on the bill that you received at the time of purchase.