Stains in teeth can be of variable types and of variable reasons, per say it may be coz of coffee or maybe coz of tea too, while some patients have chromogenic(colour) bateria which may be the reason for stains, if you have certain type of health supplements like iron syrup etc..
Next Steps
Meet your dentist on a routine basis say every 3 months for scaling with polishing
Health Tips
Rinse immediately after having stain producing food stuff