IF MY PHONE IS LOST AND I HAVE THE SERIAL NUMBER WHERE DO I ENTER IT TO LOCATE IT

If you go to the iCloud portal you should be able to go to Find My iPhone (if enabled) you may be able to locate it that way. That is probably your best bet to find it.

Here is the link iCloud - Sign in

If that does not work then you should contact your carrier, they will be able to assist further.

You will likely just use your apple ID on a computer or another device to use find my Iphone if you know you had that feature on. If you're phone is still connected to cellular or wifi it will locate where it is. If it has died it will say where it last was if the feature is turned on.

The serial number has nothing to do with remote tracking of any iOS device. You cannot track them by serial number at all.

As long as you had enabled find my iphone in your iCloud account settings on the phone before it was lost, you can track it by logging in to your www.icloud.com account. If you did not have find my iphone enabled on it, then there is no way to remote track it - none at all.