Find it with a UV light, then hit it with Nature's Miracle
It's a problem that plagues so many cat owners: cat urine in the wrong place. Anywhere but the litter box, and you have a major problem.
The first thing to do is address the cause of the inappropriate urination, of course. If you have a cat who is suddenly peeing in the wrong spot, the first thing you should do is take your cat to a vet. This is a classic sign of a urinary tract infection, which can be both painful and damaging. Male cats may also suffer from urinary tract crystals which can be excruciating and potentially fatal.
Male cats should always be neutered, and not only to prevent territorial marking (a.k.a. "spraying"). The same goes for female cats, who may otherwise urinate inappropriately as part of their hormonal cycle.
The first step to cleaning up cat urine is to find it. You may be surprised where the urine is, and how much of it there is. Cat urine will fluoresce under black light, and if you have a cat urine issue, I strongly recommend getting (or borrowing, or renting) a black light device for detecting cat urine.
Next, get rid of the urine itself. Just wiping it away with a paper towel will only get the superficial stuff. Cat urine has a near-magical ability to soak into whatever it touches, even hard surfaces like bathroom sinks and kitchen counters.
If kitty has peed on something soft, launder it immediately. The faster you get it washed, the more likely that you will be able to take care of the smell. Add a cup of white vinegar to the load of laundry, to help disinfect and remove the smell.
What you really need is an enzymatic cleaner. Nature's Miracle is the undisputed champ of cat urine clean-up. They even have a formula specially tailored to the purpose. This enzymatic cleaner will go deep into the stain and destroy the urine smell.
Be sure to follow the instructions exactly. Give it time to air dry (it can take 24-48 hours to fully work), and don't dilute the formula with water.
Nature's Miracle will work on soft things, too. Once you wash a soft item, give it a hit of Nature's Miracle to make sure you have cleaned it up.
In a pinch, if you are out of Nature's Miracle and you can't get to the store, I have found that a spray bottle filled with a 50/50 mixture of white vinegar and water helps kill the smell. It's not a permanent fix, though - the smell tends to return, especially in humid weather. And your cat will still be able to smell it, and thus will be more likely to re-mark that spot.