How do i find my lost iphone that i hadn't switch on 'find my iphone' in setting?

There are only two things you can do, call the iPhone and see if you can hear it ring, such as if it is lost at home, or look in likely places you may have lost it. There is no way to track the device if Find My has not been activated, the iPhone is turned on, and it is connected to the internet.

There is no magical way to find it. You will need to search for it, as you would search for keys, wallet, socks.