Causes for yellow staining is High flouride content in drinking water, calculus deposits or natural colour of tooth in some diseases in which there is very less enamel on tooth e.g. Dentinogenesis Imperfecta.
If it is due to calculus deposits the get Scaling and Polishing done from General dentist
If not satisfied yet then go for Teeth Bleeching.
And in extreme cases composite or ceramic veneering has to be done.
Veneer is a thin cover for front portion of your teeth and are expensive.