can be due to a number of causes like :-
Local factors like oral hygiene
(Improper brushing technique,food lodgment on the tooth surfaces,smoking, etc) or
Systemic factors like Diabetes
, Gastric refluxes resulting from dietary components rich in garlic, onions etc.
The first thing is to visit your Dentist
and get the cause of your condition diagnosed. Also get a professional cleaning (scaling/Oral Prophylaxis) done. Maintain good oral hygiene by simple procedures of brushing twice a day, rinsing and flossing after every meal. Start using a mouthwash regularly and drink plenty of water thru the day.