As stated by other members I would take the advice of the vet.
After this though there are a few things you can supplement them with to help keep teeth clean and healthy.
Dry food is preferable with all animals as wet food contains around 80% water. In the long run it saves you money putting them onto a dry food as you don't have to feed them so much. It also helps slow down tartar build up and keeps teeth stronger longer.
You have more choose with dry food and as mentioned can buy special formulated food for different problems.
A good product which I use for my dog is plaque off. It is quite expensive but seems to work. All you do is sprinkle a small amount onto the cats foood and this eventually softens the plaque, when the cat eats something hard like a biscuit (this is also why it is helpful to feed dry food) it brushes the plaque away. It takes a while to see results (about a month or so) but worked for my dog.