I know, until I had yorkies I'd never heard of brushing a dog's teeth.
Petco and other stores have tooth brushes and paste or you can get it on-line or at your vets. I use C.E.T. which is available at my vet. I like it because it leaves an enzyme in the mouth that helps kill bacteria that causes plaque. But the main thing is brushing the teeth.
I started with one of those "finger" brushes because I had more control. With a puppy I just put the paste on my finger and rub it on the gums and teeth to get the dog used to me working in its mouth.
I sit my dog on the counter where I bathe him. Let him sniff the paste first, usually they want to lick it. Then I grasp the lip with two fingers (and hold on, but not too tight), lift it up so you can see the teeth on the side. Start with a little brushing on the canine teeth and the front teeth. As you can, move towards the back teeth. If you firmly hold the lip the dog can't move too much and you can at least get started.
Again, just gentle patience and keep working on it. It will really pay off even if you don't brush more than once a week. Of course, every day would be best. If the gums bleed a little, that shows you are stirring up the bacteria so don't worry.
When you are at the vets, ask them to look at the teeth to make sure a more thorough professional cleaning isn't needed.