Cats are carnivores, not omnivores like people and dogs. They require two to three times the protein that omnivores require. There is much debate over wet or dry cat food. Both can be fine for cats. Wet food can help reduce some urinary problems that some cats have. Many vets suggest low-carb diets for cats (Atkins diets :-).
It is good to leave cats fresh water at all times. But leaving out cat food all day can led to overweight cats (especially if they don’t get much exercise).
Kittens have different nutritional needs from adult cats so feed kittens, food that is specifically made for them and adult cats food that is made for them.
Labeling requirements in the USA require that something labeled “cat food” must be 95% of more of the named ingredients: so Tuna cat food must be 95% tuna. “Diner” means it can contain 25%-95%. Cat food “with” something only needs to contain 3% of the with item, so cat food with fish can be as little as 3% fish.
Many cats will drink milk but it isn’t healthy for most of them. Most cats don’t have the enzyme lactase which is needed to digest cows milk. Some cats can tolerate milk.