Don't feel like a bad mommy-we all start somewhere! It's great that you are wanting to do research on dog food and find the best thing for your furbaby. Are there certain brands you are considering? We could probably help you decide if you could give us an idea of what you're thinking of.
A great reference is
Dog Food Analysis - Reviews of kibble Look for a 4-5 star food on there-typically a 6 star food is too rich for yorkies and intended for larger breed working dogs.