Most common cause of bad breath is poor oral hygiene. Presence of plaque or tartar causes inflammation of gums which causes bad odour. In addition presence of decay can entrap food which  rots and causes bad odour. Visit your dentist for cleaning and polishing of teeth followed by filling of decays. Use mouthwash for limited period as per your dentist advice. Other causes of bad breath  being dryness of mouth, sinusitis or any other gastric problems which should be thought at later stage.