There are several reasons for bad breath.
*Bad oral hygiene
which is most common
*use of some type of tobacco e.g. smoking etc
*improper digestion and constipation
*also sometimes the type of food u eat like garlic, spices, fish, acidic drinks etc
*dry mouth- less and thick salivary secretions
* medical problems like diabetes, GERD, lactose intolerance
* if you are wearing some type of dental prosthesis.
First thing you should do is stop intake of tobacco in any form, acidic food, garlic etc; start brushing twice daily using fluoridated toothpaste, also start using mouthwash, increase the amount of water intake. Go for a dental check up, your dentist
will tell u if you need professional cleaning of your teeth. But if the oral hygiene is proper then it may be due to some medical condition. Consulting a physician will be best.
May the joy be with you.