The first thing you need to do with a brand new mask is to clean the inside and outside of the lenses with toothpaste (not a gel toothpaste) to remove a silicone coating that comes from the factory. You only have to do the toothpaste treatment once. Then, before every dive, you should rub the inside and outside of the lenses with an anti-fog. Sea Drops
are the most popular. After you rub, do a quick dip and rinse, don't rub the lenses much afterward. You should be ready to go!