first of all u have to show ur teeth that how much yellow teeth u have.if its severe then it may be due to flourosis.if it is mild then its maya be due to staining .dpending upon etiology/reason for yellowing treatment will provided.first u should go to the nearest dentist to show the problems.

Whitening toothpaste can whiten teeth slightly by removing surface stains, such as those caused by drinking coffee or smoking. Whitening toothpaste can also be used after a bleaching treatment to help maintain results. To remove surface stains, whitening toothpaste typically includes: Special abrasives that gently polish the teeth Chemicals that help break down or dissolve stains Some whitening toothpastes contain the chemical blue covarine, which adheres to the surface of the teeth and creates an optical illusion that can make teeth appear less yellow. When used twice a day, whitening toothpaste can take from two to six weeks to make teeth appear whiter. Whitening toothpaste that contains blue covarine can have an immediate effect. Although whitening toothpaste is generally safe for daily use, be careful to follow manufacturer recommendations. Excessive use of whitening toothpaste can damage your tooth enamel over time. Keep in mind that whitening toothpaste can't change the natural color of your teeth or lighten a stain that goes deeper than a tooth's surface....Consult dentist for further help Regards