If your mobile phone has been lost or stolen the one sure way to make sure the handset is unuseable is to contact your network operator and have the phone put on a blacklist. To do this you need the IMEI number.
How can I find it?
If you have disposed of the packaging dial *#06# on almost any model of phone and the IMEI number will be displayed. Usually this is 15 digits. Alternatively somewhere on the inside of the phone the number will be displayed.