Needs to be evaluated properly,as halitosis depends upon so many things. The causes of bad breath include certain foods, alcohol or cigarettes, poor oral hygiene, periodontal disease, diabetes, dry mouth, sinus or throat infections, lung infections or abscesses, kidney/liver failure, gastrointestinal issues and severe dieting. The key to a clean, fresh mouth is optimal oral hygiene ,antibacterial toothpaste, antiseptic mouth rinse, tongue brushes or scrapers and interproximal cleaning devices are also equally important and Cleaning your tongue with  tongue cleaner on the back of the toothbrush for cleaning both your teeth and tongue. After tooth brushing your upper and lower teeth with an antibacterial toothpaste, flip the toothbrush over to the tongue cleaner and place the tongue cleaner in the posterior region of the tongue and move it forward to the anterior section of the tongue. After you have scraped that portion of the tongue, rinse the tongue brush off with warm water to remove any odor causing bacteria. 

You need to get a check up done to determine the exact cause. The most common reason is deposits on teeth/gums and food lodgement in between teeth. If this is the case, then you have to get cleaning done. However, you need to see a dentist to find the exact cause.