Nothing. The enamel of your test is fully formed before the tooth erupts into your mouth. After the tooth erupts there are no foods or supplements that will "thicken" the enamel. Keep teeth clean and avoid acidic foods and beverages. You may also want to consider a fluoridated toothpaste and/or a Fluoride rinse. Children benefit from Fluoride supplements if there is no Fluoride in your water supply.
Nothing you can do. Enamel is non-vital and does not grow after your teeth erupt, so there is no way to make the enamel on your teeth thicker.