Brushing your cat's teeth should be a regular part of his grooming routine. Over time, plaque and tartar will build up on his teeth and will need to be removed, so start brushing at an early age to get your cat used to it and to avoid extensive buildup.
Before beginning to brush, open your cat's mouth and take a careful look at his gums and teeth. The gums should appear pink and healthy, and there should be no broken or loose teeth. When you're ready to get started, put a tiny bit of cat toothpaste on a brush and start brushing, one tooth at a time. Only the outsides of the teeth need to be brushed, because kitty will take care of the insides with his rough tongue. Be sure to gently brush the gums, too.
Taking care of your cat's teeth is important, as neglecting this care can lead to teeth decay and gum disease. You should brush his teeth at least once a week, but more often is even better. You may want to purchase a special "dental" dry cat food, which has been shown to help keep teeth clean between brushings.