I am having my braces now. should i brush my teeth immediately after drink tea in morn, evening. my teeth is little yellowish due to my water.

Dental evaluation: Cetainlt check with your orthodontist and ask about tooth bleaching.

Yes and no: to have a cavity, you need 3 things (acid, carbs and bacteria). If it's just plain tea with no table sugar, you dont need to brush it since there is no carbs present, just rinse your mouth with water to minimize staining. Regarding gum disease, one of the properties of black tea is mildly antiseptic and bacteriostatic (cool, isn't it?), so it might actually help fight gum disease :)

1/2 hour: You don't want to brush, with or without braces, for a 1/2 hour after eating or drinking anything acidic. The acid leaches calcium out of the tooth surface, and it takes about a 1/2 hour to remineralize. Best to rinse with water after eating/drinking, and brush 2x/day (before bkfst, before bed) for 2 full minutes by the clock. You may want to use pea-sized portion of whitening toothpaste.