How do I treat bad breath?

Oral hygiene. Most bad breath is due to crud on the mouth, between the teeth or on the tongue. A good dental flossing with running it over the back of the tongue cures most of these. If it does not, and you are not eating anything like onions or garlic, look for a problem in the upper aerodigestive system.
Find out why. There are many reasons for bad breath. It can be related to what you've been eating, e.g. Garlic, onions. It can also be caused by esophageal reflux (belching), dental disease, and sinus infections. Your doctor and dentist can help you sort it out.