Make sure iTunes is installed on your PC/MAC and make sure your iPhone is turned off.
Put iPhone into recovery mode doing the following:
>iPhone 8 and later - Hold the right side button whilst immediately connecting your iPhone to the PC/MAC
>iPhone 7 and 7 plus - Hold down the volume down button whilst immediately connecting your iPhone to the PC/MAC
> iPhone SE (1st generation) and earlier - Hold down the Home Button whilst immediately connecting your iPhone to the PC/MAC
>Keep holding button until you see a laptop icon come up on your screen, if you see the passcode screen you need to turn off your phone and try again.
> Locate your phone in iTunes
> press on it and choose "Restore"
> When process finishes your device will show the setup screen
> Disconnect iPhone from computer and setup your iPhone.