In most the cause of bad breath is the accumulation of food debris and bacterial plaque on teeth and tongue.
Oral malodour could be caused by: the foods you eat (garlic and onions, in particular), poor oral hygiene, gum disease, diabetes, dry mouth, alcohol or cigarettes, sinus or throat infections, lung infections, kidney/liver failure and gastrointestinal issues.
Whilst some people are unaware of their bad breath while others are very concerned that it is socially offensive. It's difficult to assess how your own breath smells, if you are concerned about Oral Malodour consider seeing a dental professional to confirm if bad breath is an issue for you and gain advice as to how to treat it.
The best way to prevent bad breath through the accumulation of food debris and bacterial plaque is to brush with a Fluoride toothpaste like BioMin® F toothpaste and floss or use interdental brushes to clean between the teeth every day.
There are also other techniques to freshen your breath, which your dentist or doctor can discuss with you.