is caused due to poor oral hygiene
I would suggest you to get your teeth cleaned by a dentist
Use soft bristled toothbrush and clean your teeth involving all aspects of teeth.
Rinse your mouth after every meals.
Replace toothbrush in every 6 months.
Maintain oral hygiene well.
Take plenty of fluids and nutritious diet.
Fruits and leafy vegetables has to be included in diet.
Hope this helps.