There are many reasons for bad breath, but the number one reason is undoubtedly insufficient oral hygiene measures. This begins at home with daily standard brushing and flossing and extends to getting regular cleanings and checkups at the dentist.
Most daily brushing and flossing removes the majority of microbes that create the bad smelling compounds in our mouths. Daily brushing with a fresh, soft toothbrush whose bristles aren't bent and less effective is most important part of mining good oral hygiene that can keep away bad breath. Using small circular motions to brush all surfaces of the teeth -- gum line, front of the tooth, biting surface, as well as inside surface toward the tongue, are all important to address. Ideally, we would brush after each meal, but realistic time constraints in our lives means that brushing twice a day is the standard expectation by dentists for their patients. In addition, daily flossing between each tooth will remove any food material that brushing was unable to dislodge, further reducing substrates for bacteria to turn in bad smelling compounds.
Despite our best efforts to brush and floss daily, over time hard tartar (calculus) forms on teeth at a rate that varies genetically. Ultimately, that tartar must be removed manually by a dental hygienist to get the teeth truly clean. Your dentist and hygienist will tell you how often you need to get dental cleanings. The standard frequency is every six months, however, due to genetic differences, some people should get them every three months.
These are by far the most important steps you can take to keep your mouth smelling and feeling great.