Hi there,
Firstly, go for scaling (teeth cleaning) which scales away the Plaque (yellowish substance) if the yellow stains r superficial.
Secondly, if u r not satisfied with the Scaling results u can go for Teeth Whitening (bleaching). Bleaching is prescribed when the Stains r deep or intrinsic. This will give u good results hopefully.
Next Steps
Consult to a dentist.
Health Tips
Avoid consumption of high amounts of Caffeine and Soft drinks.