how do i find my lost iphone

If you had setup an iCloud account the iPhone, and IF you had enabled the setting "find my iPhone" under that account's settings, you MAY be able to track it by logging into your iCloud account from a web browser.

Otherwise, there is no way to track it remotely.

In order for find my iPhone to work, along with the account & settings I already mentioned, the device must be powered on and it must have a live data connection (wifi or cellular, either will work). If it has been powered off, or the battery run down, or if anyone has restored it as new (thus wiping out your iCloud account and its settings), then it will not be trackable.

The only way to locate any lost/stolen iPhone is through Find My Phone or a similar app. However, this requires that Find My Phone be setup/activated, on your phone, before it was lost/stolen. You would then login at iCloud.com & try to locate it. This requires the phone be turned on & have an Internet connection. There is no other way to locate a lost/stolen iPhone. Apple can't/won't help you, nor will your carrier. Report the loss to the Police, your carrier & Insurance company. Change all of your passwords.