The best pet toothpaste contain enzymes that help control plaque. Try to avoid toothpaste's with baking soda, detergents or slat sometimes found in human pastes. Fluoride may be incorporated to help control bacteria. Rather than placing the paste on top of the brush try to incorporate it between the bristles. Step three is to get the brush with paste into your dog's mouth and all the teeth brushed.
Most dogs accept brushing if they are approached in a gentle manner.
If you can start when they are young, it's quite easy, but even older pets will accept the process. Start slowly, you can use a wash cloth or piece of gauze to wipe the teeth front and back in the same manner you will eventually be using the toothbrush. Do this twice daily for about two weeks and your dog should be familiar with the approach. Then take the pet toothbrush, soak it in warm water and start brushing daily for several days. When your dog accepts this brushing, add the pet toothpaste.