Diet and poor oral hygiene are the most common causes of bad breath. It may be simply corrected by using your toothbrush to brush your tongue. Food, coffee and bacteria collect on your tongue and create a coating which causes odor. Bacteria produce sulfides which may require more extensive measures to combat the problem.
Another cause for bad breath is a tonsilith. Tonsilith or "tonsil stones", are collections of various matter bound together in your tonsils. This matter is comprised mostly of food particles, dead cells, and oral bacteria which can form into hardened white balls and cause bad breath. If the tonsiliths have not hardened and matured you may be able to simply gargle the bacteria away with salt water. If you can reach the tonsil stone with your toothbrush, try to scrape them away. You can also try pressing a q-tip down on your tonsils which sometimes pop the tonsiliths out of place. In most cases tonsiliths will eventually remove themselves but if not be sure to go to a doctor just in case your tonsil stone is more severe than it seems.
Persistent bad breath or a bad taste in your mouth may be warning signs of gum (periodontal) disease. Gum disease is caused by the buildup of plaque on teeth. The bacteria cause toxins to form in the mouth, which irritate the gums. If gum disease continues untreated, it can damage the gums and jawbone. This should be evaluated by the hygienist and dentist to help determine the origin of the problem.