Is your cat spraying? Or is he or she just peeing in the wrong place? Is it a medical problem, or a behaviour problem?
If the cat is painful, not eating, or straining without any urine coming out, it is important to see a veterinarian right away. If you are in any doubt, call your local veterinary office. If your cat is blocked and cannot urinate, this is an EMERGENCY.
Here are some useful resources:
Feline Lower Urinary Tract Disease This introductory article links to other articles, including feline interstitial cystitis, feline house-soiling, and urinary blockage.
Basic Litterbox needs, from the Indoor Pet Initiative
Conflict Between Cats is a common cause of inappropriate elimination.
Feliway: A pheromone used to treat urine marking and other stress disorders in cats.