There are many articles, videos, etc about what should (and shouldn't) be in your cat's food. It can be quite confusing but is a good thing to know.
The first thing you should do is look at the label on the bag, can or pouch. What are the top ingredients? Are there by-products? Is BHA/BHT present (NOT a good preservative to have in the food)? Is corn or wheat in the food (if so, where on the list?)? Here are two example cat foods of similar price and their first ten ingredients.
Chicken Soup for the Cat Lover's Soul Adult Formula
Chicken, Chicken Meal, Cracked pearled barley, Chicken fat, Ocean fish meal, White rice, Potatoes, Oatmeal, Millet, Natural chicken flavor
Purina ONE Chicken & Rice Formula
Chicken, Brewers rice, Corn gluten meal, Poultry by-product meal, Wheat flour, Animal fat, Whole grain corn, Fish meal, Soy protein isolate, Animal liver flavor
So, which one looks better? Well, let's compare. The top ingredient in both is chicken, which is good to see because it is a muscle meat. In the Chicken Soup for the Cat Lover's Soul, the next ingredient is chicken meal (the dried form of chicken) which is GREAT to see because it also is a muscle meat. In Purina ONE the next ingredient is brewers rice which is NOT so great to see because it is one of the lowest quality forms of rice. In Chicken Soup for the Cat Lover's Soul, the third ingredient is cracked pearled barley which is a great grain choice because it is a whole grain that is also a slow burning carbohydrate source and is high in protein. Purina ONE has corn gluten meal which is known to cause allergies in many animals and is of little nutritional value. Forth in Chicken Soup for the Cat Lover's Soul is chicken fat which is a great form of fat for the food. Fourth in Purina ONE we have poultry by-product meal which is not a good form of meat (if they wanted us to know what meat it was they would have listed it as chicken by-product meal or turkey by-product meal, etc). Fifth in Chicken Soup for the Cat Lover's Soul is ocean fish meal which is dried fish and is good to see because it is a high quality meat that is a good source of protein and omega-3 fatty acids. Fifth in Purina ONE is wheat flour which is another cause of pet allergies and is also of little nutritional value. Sixth in Chicken Soup for the Cat Lover's Soul we have white rice which is a sound inclusion because it is one of the better types of rice to put in cat food because it is a good carbohydrate source and is high in protein. Sixth in Purina ONE we have animal fat: um, where do I start with this one? Well, "animal" is not very descriptive and if it were beef, or chicken fat they would say it was. It is MUCH better to see a specific animal listed so you know the source. Seventh in Chicken Soup for the Cat Lover's Soul we have potatoes which are a great form of starch and is great to see because they are a highly digestible carbohydrate source that is not likely to cause allergies. Seventh in Purina ONE we have whole grain corn which is again, NOT good to see (for the same reason as the corn gluten meal). Eighth in Chicken Soup for the Cat Lover's Soul we have oatmeal which is another great grain to include because it is a whole grain. Eighth in Purina ONE we have fish meal which is not bad. Ninth in Chicken Soup for the Cat Lover's Soul we have millet which is another great grain choice and is a sound inclusion because it is high in protein. In Purina ONE we have soy protein isolate which is bad because cats don't need soy (it is a cause of allergies in cats). Chicken Soup for the Cat Lover's Soul uses natural chicken flavor for their kibble and Purina ONE uses animal liver flavor. I would prefer to see a specific type of animal listed instead of just "animal".
As I said before, these foods are very similar in price and there is a HUGE difference in quality so just read the label on any food(s) you are considering! Now, Purina ONE is NOT a BAD food (there are MUCH worse), you can just do better!
Now look on the "Guaranteed Analysis". What percentages of crude protein, crude fat, and crude fiber are in the food? A lot of people think that the higher the percentage of crude protein, the better the food but that is not always true. While a higher percentage of crude protein is good to see most of the time, it depends on where the protein comes from. If it comes from soy, by-products, etc then it is not a high quality protein and therefore, not as good as a food that is lower in protein but has a high quality source such as chicken, fish, duck, or turkey (or a meal of any of these) and barley, rice, millet, potatoes, etc. So for instance, a food that has 34% crude protein that has by-products, soy and corn is not as good as a food that has 30% crude protein that has chicken, fish, and barley. Crude fiber in most cat foods is between 1%-5% which is fine (this number can vary but for the most part, they all have similar amounts). The crude fat in cat food is usually somewhere between 12%-20% but can be more depending on the food. The moisture content will vary depending on whether you feed dry, or wet (I have given dry food examples). The dry food moisture level is usually about 10%-12% and the moisture content in canned food is usually about 78%. I recommend feeding a combination of wet and dry foods because cats tend to not drink enough water and can get dehydrated, but all wet or all dry is also OK. If you feed all wet, make sure to keep the teeth clean. If you feed dry, also make sure to keep the teeth clean and also make sure your cat gets enough water.
Now I know that there have been a lot of pet food recalls lately and it can be hard to pick a brand to stick with because we don't want a brand that has been recalled but from what I have read and seen, there are VERY few brands that have not ever been recalled at some point so just stick with brands that have a good reputation and keep up with what is safe and what is not.
Every cat deserves the best food possible!