I highly recommend "Science Diet" pet food. I've used it on my 1 year old pomeranian since I got her. If your yorkie is a puppy, get the small kibble or the soft food from "Science Diet." You might want to continue with the small kibble for adults after puppyhood since the dog's mouth will always be small.
Most dogs do just fine with commercial dog foods available at your local store. Higher quality dog foods can be purchased from pet stores or your local vet. If you select a dry food for your Yorkie, select one with small pieces to make it easy to chew.
They do the very best on Kibble Selct by Pruina it gives them a complete balanced diet in a bag and do not ever give the dog table scraps for some reason it will make those little guys sick every time.