How do I find my lost iPhone?

If you are saying that Find My was not enabled on your iPhone.....before.....it was lost, there is no way for you locate the phone online.

All that you can do at this point is retrace your steps and physically look for the phone at shops or restaurants that you might have visited.

If you did not have FindMy turned on, then there is no way you can track the device.

If your device did have FindMy turned on, then you will have to wait for the device to make a cellular connection, or another Apple device to be within bluetooth range.