Thanks for asking the query,
Yellowish teeth can be because of various reasons , poor oral hygiene due to plague
and calculus deposition , exessive comsumption of tea , coffee , tobacco.
It can also be because of hereditery conditions , illness , excessive flouride intake , use of certain medications .
Go for complete mouth scaling and polishing , ideally scaling should be done after every six months . whitening of the teeth can be done by bleaching , veneers and bonding techniques if required .
Brush your teeth twice daily , develop habit of flossing the teeth , avoid use of substances that causes extrinsic stains.
Hope this helps out.