Brushing and flossing twice a day is the best remedy dear, you can buy sensodyne whitening toothpaste from any chemist shop, this paste has prescribed by my cousin's doctor as her problem is also same and I would suggest you to try a home remedy too which I am gonna sharing with you - mix a teaspoon of baking soda with a few drops of water then apply it to your toothbrush it will take some time but gradually you will see the difference for sure with in a month. Have lots of vegetables and fresh fruits in a day. Take care.