Diarrhea. There are numerous causes. The simplest is worms, usually roundworms or tapeworms. Roundworms look similar to spaghetti while tapeworms look like rice (under the tail) or can be long & flat. You want to get the correct medicine for whichever worm it is. Further, tapeworms are caused by the cat eating fleas, so you should also treat it for fleas. Tapeworm & roundworm meds are available in some stores, online and, from your vet. Other causes for diarrhea, for which there are various meds, can be determined by a vet.
Fleas. The most commonly-used flea meds are Frontline, Advantage and Revolution (Revolution can only be obtained from a vet). We do not recommend cheap flea collars, but Seresto brand collars are good and last for up to 8 months. Even indoor cats get fleas, which are carried inside by dogs or people on their pant cuffs. So it is advisable to treat with flea meds monthly or use a Seresto collar, at least from the warmer spring months until winter when fleas do not thrive as well. Cats—and especially kittens—that have lots of fleas, become anemic and lifeless and may even die.
Urinary problems. If your cat is straining to urinate or goes in and out of the litter box frequently, but does not produce much urine, there is a good chance it has urine crystals which are blocking it up. If this continues for long, it can be fatal. A vet can take an x-ray to determine how severe it is. Sometimes they will recommend a calming medication because they believe the cause is nervousness. But that often does not cure the problem at hand. The problem is usually alleviated by catheterizing and ridding the body of the crystals.
Not eating. And possibly losing weight. The most common reason for this is oral or dental pain. Your cat may have a lesion/sore on its tongue. You may find that eventually (after seeing a vet), the simple cure is giving it Lysine for this, which you can get on line, at some stores or from a vet. It is not expensive. The cat may have one or more abscessed teeth, which will require extractions, just as with people. Or they may have inflamed gums, which will require having a dental cleaning. Or they may have a gum disease called stomatitis, which requires periodic (every 4-12 weeks) medication. Drool around the jawline is another indicator that your cat has a dental issue. There are many other internal reasons for lack of appetite. Bottom line, see a vet.
Lethargy/lifeless. There are many causes for this. Look at the inner top of the ear. The skin should be flesh or pinkish colored. If it is yellowish, it indicates a liver problem. Lack of appetite, lack of stool, less interest in walking or doing routine things, see a vet. If it is sickly and also hiding, see a vet.
In general, if your cat seems sick, see a vet.
Litter box. If your cat isn’t peeing in the litter box, try placing it in an area where there is less traffic, fewer people going near it. If you have a dog, place it where the dog cannot bother the cat when it uses the box. Try adding an additional box in another part of the house. Scoop the box at least once a day, ideally twice a day. Many cats will not use a messy box. Try a new brand of litter. Always have multiple boxes to accommodate more than one cat.
Scratching furniture/walls/door frames. Ideally, you can find a scratching post or scratching item in a pet shop that the cat will use instead. Sometimes it helps to sprinkle catnip on the post to make it more inviting to use. Some cats like to scratch a throw rug. You can learn to trim their claws (do not clip the darker or pink area close to the base) or take them to a groomer to trim, which is usually inexpensive. We do not de-claw our cats, though some come to us that way. It is especially cruel to de-claw an adult cat.
Scratching people. Some cats get over-stimulated after being petted for more than a few strokes. Knowing this, do not push its limits. Some cats do not like some people. Some cats do not like children or are particularly sensitive to being picked up, especially improperly, or being handled roughly. Some cats get agitated by loud noises, like fighting and music.
It’s always good to google a few cat sites to see what other cat owners have experienced and what has helped. Also check out Jackson Galaxy’s site.