One sad fact of pet life is that there are some pets who become strays, either because their owners abandoned them, or because they got lost. Many stray pets suffer from malnutrition, either eating the wrong food, or not enough food. How can you, a concerned pet lover, help stray cats?
The first thing you must do is try to find the cat's owner. This is because in most areas you cannot legally take someone's pet into your care as this could be mistaken for theft. If the kitty is friendly you can check it for a collar or identification, such as a tattoo. Some cats have microchip ID, which you will not be able to see, but a veterinarian or shelter can check for no charge. Ask in your neighborhood if anyone knows who owns the cat. Be sure to call your local animal shelter and report finding the cat. They may have a "Lost Cat" report on the kitty, and hopefully its owner will be eager to get it back. Sometimes people dump their neighbor's cat in another part of town simply to get it out of their neighborhood.
If there is no known owner, file a "Found Cat" report with the shelter and decide what you want to do. You may elect to take the cat to the shelter itself and leave it with them, or you may decide to "keep" it and care for it. If the cat is seriously malnourished, it should be taken to the shelter who will place it under veterinary care while they try to find its original owner.
Some people commit to undertake the responsibility of feeding the cat, others will welcome it into their home. Generally you have to allow two weeks after reporting finding the cat before you can really consider it "yours".
Feeding a Stray Cat
Malnourished could mean either the cat is either underfed, fed the wrong thing, or fed too much of a bad thing. If it is a kitten it should be feed kitten food, usually dry food is left available throughout the day, and a small amount of canned food offered 1-3 times a day depending on the kittens age. Adult cats who are young and thin can be given kitten food to help them gain weight, but on the whole an adult should have only adult food, or senior cat food if they are older. Generally speaking the foods sold in grocery stores are adequate, but contain a lot of filler so truly are not best for cats. If you see the word "by-products" on an ingredient list, you can be sure the food is lower quality. Similarly if you see corn in any of the top ingredient positions the food is low quality; cats are carnivores, corn is very hard for them to digest.
If the cat was starving you should introduce food slowly, limiting the food intake because if a starving cat eats too much food at once it could die due to stress on the stomach. In some cases you may even want to start out with very bland food such as baby pablum, mixed with a bit of the canned cat food. The canned food should be a chicken pate formula, not anything that is chunks in gravy.
Note that if you try to feed a starving cat and it vomits perhaps there is something wrong with the cat, such as a bone stuck in its throat.
Veterinary Care for a Malnourished Cat
If you have committed to the ongoing care of this cat, it will need veterinarian attention. Most of this should be done after the two week waiting period for an owner to come claim the kitty. Have the veterinarian examine the cat, worm it, vaccinate it, and desex it. Deworming, treating for fleas, or ear mites, could be done earlier if needed, and are certainly concerns of stray cats. Spaying or neutering prevents many negative behaviors and reduces (or eliminates) the cats risk of many health problems and cancers.
Care for the Cat
Some people consider themselves the care givers of a stray cat, but if you "really" want to care for the cat, you should consider yourself the "owner". It is not enough to put food and water down, and otherwise let the cat fend for itself. Cats need real homes, with real owners. They need warm places to sleep and cozy laps to cuddle on. If you really want to help a stray cat, befriend it and welcome it into your life.