I use the smallest human baby toothbrush they sell at the drug store with the Nylabone dog toothpaste that fights tartar buildup.
If you're just starting, you could also try first using a small cloth over your finger to get your puppy used to his mouth/teeth being handled. Do this daily before using the toothbrush. Or...you could jump straight to the toothbrush. Whichever makes your life easier.
Just make sure you don't use human toothpaste
