Question

How to set ringtone in Moto G.I could not find it in settings.

Answer 1

Have you checked in settings/sound & notifications
next to General you will find a settings icon, click on it, there you will find phone ringtone option, you can set ringtone from there.

Answer 2

You can finish this task via settings. 

setting->ringtones->choose a sim->changes ringtone

One more thing you should note is that the size of ringtone is less than 300KB. If mp3 file is too big to be set as ringtone, you should trim mp3 files to smaller size.  Check this article on how to trim MP3 on Windows Media Player.