You are welcome! PetSmart and Petco are petstores, but they are big box stores, and you'll want to look for smaller upscale pet boutiques.
I know very little in the grand scheme of things, but I always try to help in the ways I can.
There are others here who know much more than I do.
Corn is not inherently bad, but the corn products dog food manufactures use are merely fillers. Fillers hold little to no nutritional value and are the remains of corn after the nutritious part has been removed. Corn is not a natural part of a dog's diet, and is also one of the top allergy ingredients. I personally stear clear of dog food containing corn products.
"By-product" is something you always want to avoid. Meat by-products are parts of the animal not including meat.
I personally like to see the first 2 (or more) ingredients to be meat products, with at least one being a meat meal. If only the first ingredient is a meat product, but it's a meat meal, I can go for that. A shorter list of ingredients is a good thing.
Here is a website that you might find helpful.
Dog Food Analysis - Reviews of kibble Dog food is rated and reviewed.