The discoloration .. could be either extrinsic ie. due to diet ( colored foods, tea/coffee) and improper cleaning or intrinsic which may be due to some enamel defects or too much fluoride conc jn water etc . Which part of the country u tay ? Share a photograph so that , the discoloration pattern can be seen .
Next Steps
Visit a pediatric dentist who would be able to recommend you the best possible treatment as per the age and dentition