Lousy breath is way worse than yellow teeth. Your teeth can be healthy without being as white as a sheet of A4. Bad breath usually is a sign of bad hygiene BUT, oddly enough, it can also be caused by certain dental products (e.g. Crest products give me bad breath after an hour or two whereas Colgate products do not).
For those who are wondering about hygiene though:
Brush at least twice a day (preferably after each meal) and floss regularly. Don't be afraid to edge into the flossing slowly. For the longest time I never flossed and so when I started doing it regularly my gums were extremely sensitive and tended to bleed.
Make sure you use a bristle stiffness that is comfortable for you. Everyone's gums are different and so you may be better off with a softer-bristled toothbrush or a stiffer one. A Sonicare is not a magic wand (well...not DENTALLY anyway) and people are often surprised to learn that I use a good old-fashioned manual toothbrush
For white teeth I find that splitting the difference between professional treatments and gargling hydrogen peroxide usually works the best. Buy a tube of Rembrandt Deeply White (or Intense Stain depending on how yellow your teeth are) and the matching mouthwash. Both contain safe levels of Urea Peroxide but much higher than your average "whitening" products. (i don't like strips or trays as they are often too powerful and can cause horrific, long-term inflammation of the gums and intense sensitivity)
Also purchase a tube of Sensodyne ProNamel. Brush, floss and gargle with the Rembrandt products at night and then in the morning (prepare for BAD morning breath BTW, especially with the mouthwash) brush with the Sensodyne to ward off the sensitivity that can develop with daily use of powerful whitening products.
Get in the habit of running your tongue over your teeth. If they feel fuzzy, rough or anything but smooth and glassy then you're due for a brushing. The key to getting your teeth healthy and white is consistency. It doesn't hurt to be a little neurotic either. If you carry a bag or backpack during the day get in the habit of keeping a "spare" toothbrush and a travel-size tube of Colgate in one of the pouches (I know, I know. I use 3 types of toothpaste, right? I find colgate has the best, longest-lasting "clean" feeling, taste and smell. In other words, perfect pre-date toothpaste).