The question you have asked is most commonly asked question. Well! you have not mentioned wether it became yellow or it was like this from before or always.
* If it became yellow now, that means there are deposites and stains on your teeth, in this case you should go to your dentist
for oral prophylaxis or cleaning of your teeth followed by polishing.
* If it was like this from before and you wanna get whiter & sparkling one, you can go to your dentist and on your dentist's advice can go for bleaching of your teeth.
Ther are two types of bleaching :- 1)Office Bleach, done by the dentist in the clinic. & 2) Home Bleach, the bleach and the template is given to you by the dentist to use it at home.
Don't brush your teeth vigrously, it might cause wearing away of enamel (the outer layer) of your teeth. some stains are intrinsic which doesn't go by brushing and needs bleaching.
Some times yellow stains are because of
a)Fluorosis, type of defective enamel causes by exess amount of flouride in water of that perticular area. In flouride staining bleaching or laminates gives good results, depending on the case.
stainings, type of intrinsic staining cause in an individual when the pregnant mother consumes tetracycline drug. In such cases again Laminates & bleaching helps, depending on the case.
I hope this information will be helpful. Contact your dentist, I am sure he can give the best possible advice after having a look on the situation.