Dogs and cats have diphyodont teeth. That means they go through two sets of teeth, just like we do. Puppies and kittens have sharp deciduous (baby) teeth that they normally lose and replace with their adult teeth by the time they're about six months old.
As they're losing their baby teeth and growing in their adult teeth, puppies (and even kittens) like to chew...a lot. Make sure to give them an outlet for their teething, or they might start using your shoes and furniture. Make sure the things they chew on are safe for them, and discard toys once they start breaking into small enough pieces to swallow.
Just as important as providing them with things to chew on, start conditioning them to having their teeth brushed. While it won't really have a vast effect on their baby teeth since they'll lose those anyway, it will get them used to having their mouths messed with. Start simple, and just use your finger (or a finger brush) to "go through the motions". Once they get used to it, you can introduce a dog/cat toothpaste (NOT human toothpaste!) and a toothbrush.
Brushing your pet's teeth everyday will provide benefits similar to you brushing your teeth everyday: less tartar, better breath, and better oral health overall.