Go to account.microsoft.com/devices. If you're prompted to sign in, use the same Microsoft account you used to sign in on your phone.
Choose the phone you want to erase, and then click Find My Phone.
Click Erase.
If you're absolutely, positively sure, select the I'm ready to erase my phone check box, and then click Erase.
[h=3]Note[/h] It's a good idea to let your mobile operator know that you've lost your phone. If you have credit cards on your lost phone, contact your bank to report them as lost.