Dogs like people require to have clean healthy teeth and gums. In dogs dental diseases can spread to the heart and in the worst cases can cause hearth problems and even death. At the very least keeping a dogs teeth clean will prevent nasty "dogs breath".
Your vet will be able to recommend a good suitable toothpaste for your dog or some people used salt diluted in slightly warm water. There are different sized brushes designed for dogs too. Use a large toothbrush for larger dog breeds or if you have a small or medium sized dog then use a small toothbrush.
People often give dogs bones or other various toys or chews designed to help your dog maintain healthy teeth and gums but these alone are not enough, you should still establish a routine of brushing your dogs teeth yourself. Never give a dog chicken, fish or lamb bones to chew on as these are quite brittle and may splinter or become lodged in the dogs throat.
When brushing your dogs teeth it is important that you are both relaxed. A dog that is playful or fearful may be hard to get him to stay still for you. Make sure that the dog is at a level that is comfortable for you and allows you easy access to your dogs mouth.
Put a small amount of toothpaste on the brush and gently lift the upper lip on one side. Starting at the back move the toothbrush in a circular motion slowly and calmly. You should only need to clean the outer surface of the teeth as the dog will clean the inside with his tongue. Make sure to also clean the gums especially where the teeth meet the gums.
If your dog will not stay still you can try holding his muzzle with one hand while brushing with the other. Do not be too forceful while doing this as it may upset or anger the dog and he will always be in this stae while having his teeth brushed.
Some dogs do not like the normal toothbrushes and instead you may need to experiment with using a rubber tip which goes on the top of your finger and can be used like a toothbrush. Other dog owners prefer to use a cloth or gauze to clean their dogs teeth.
If your dog does not like their teeth being brushed then you can use cleaning time as a game for the dog. Establish a routine where by you brush his teeth and then he gets his evening meal. He will see getting a meal as a reward for having his teeth cleaned and in time will be eager or at least will put up with getting his teeth cleaned.