Attach clear photographs of effected area. If its because of any deposit on tooth then you may need scaling or professional cleaning of teeth. If it's naturally yellow in color then you can go for bleaching. But need to check before diagnosing anything. Consult doctor online or offline for better guidance and treatment options.

Hello... To start with we need to know why your teeth have yellowish color. Normally everyone has a slightly yellowish shade and there is nothing to worry if it is naturally . If it is because of plaque and calculus and extrinsic stains (tea,coffee) , dental cleaning can help you in that. But if it is because of some intrinsic problem you can get in clinic bleaching done, it can last for few years depending upon your shade and care.

For any external stains on teeth it is recommended you see a dental hygienist for a thorough clean and polish. Internal stains can be a little more stubborn and can be treated by your dentist with a variety of different whitening methods.