Yellow discoloration caused due to various reasons. Some most common are like Dental Flurosis. In this case u have to visit dentist to check out the severity of the problem and get treatment accordingly.
Second one are like tobacco, Gutakha stains, Tarter deposits, In this case you need to go through Professional Scalling and Polishing treatment.
Next Steps
Visit Expert Dentists and get accurately diagnose your exact problem and Treatment.