First a foremost you will need a trip to the vet to check the mouth for gum disease, broken/diseased teeth, tooth abscesses, or any oral masses.

Normal dog breath should not "stink", at least, not to the point of owners complaining.

After a likely dental cleaning, you can maintain your pup's teeth by daily brushing (once monthly brushing at the groomer IS NOT adequate and does nothing except waste your money), daily oral rinses, daily oral wipes, daily dental chews (not the cheap OTC stuff you find at walmart but the legit dental products like CET Chews), etc.

Once the plaque/tartar and gum disease is present, the ONLY thing that will fix it is a thorough dental cleaning under anesthesia at your vet.

Also, forgot to mention... "stinky breath" can be an indication of other severe underlying disease processes, like kidney disease, that may have nothing to do with the mouth at all. Always best to get it checked at the vet!!!!