Yellow coloration may be due to following reasons
A) excess of tea or coffee
B) bore well water hving lot of flourine in dat
C) milk wid bournvita or other coloured stuff
D) habits like pan mashala smoking etc
Scaling is d treatment of choice for removal of stains .still if you need extra whitening den go for bleaching.
Kindly get scaling and polishing done and if you want you can also get bleaching done. The cost of treatment varies from dentist to dentist, area and speciality of your dentist you are visiting. Kindly visit your dentist and get the required treatment done. Brush your teeth twice daily using correct brushing technique to maintain your oral hygiene.
Brush atlst twice daily, at night brush with baking soda and rinse your mouth with lemon water, rub a slice of lemon, chew few leaves of basil in the morning, use mouth wash having hydrogen peroxide, with this you need to visit a dentist you may need bleach.
Foods/drinks- Coffee, tea, cola can stain your teeth.
Tobacco use, Poor dental hygiene. ,Medications r some other causes.
For cavity in your teeth you need to visit a dentist personally and get it filled.