To control a flea infestation, adult fleas on the cat must be killed and re-infestation from the environment prevented. When treating fleas, don't use products suited for dogs - these can be toxic to cats.
There are a wide range of products available to keep fleas at bay, including collars, shampoos, sprays, foams, powders and tablets. Your vet will be able to advise you on the most safe and effective treatment for your cat.