As you can see from the multiple pictures on the internet of cats chewing on a toothbrush, if you start with a tasty toothpaste, kittens and puppies can be trained to accept the feeling of a toothbrush in their mouth. In fact, in my experience, I have found that most puppies and kittens are quite keen to chew on things and if you start with putting something tasty on the brush and let him play with it, it is a small step to move from that play to purposeful tooth brushing.
Since you have probably missed that golden opportunity for your pet, the next technique is simply to put some tasty substance (tuna, salmon, canned food, peanut butter, cheese whiz) on your fore finger finger and after petting a your pet’s cheek a few times, slip your index finger with the treat between the cheek and the back molar. Initially, your pet may be surprised with this tasty interruption, but over time, you will be able to do a bit more than just wiping your finger, moving eventually to a few gentle massaging strokes along the gum line.
Once you are able to do this without resentment, you can move on to some flavoured tooth paste products. For long term benefit you should try to move on to using a knitted glove and then eventually a soft small headed tooth brush. The best prevention that I have found is a product that releases oxygen ions into the gum tissues. The infection that establishes itself in those gum pockets hates oxygen so these oxygen ions naturally select for the healthy bacteria. We carry a product like this that can be purchased on-line direct from our 
home care page.