Baking soda mixed with a tiny bit of water for a fine paste.
Brush and floss your teeth at least twice daily.
Switching your toothpaste to a whitening toothpaste. (Such as Aquafresh extreme clean)

Don't replace your tooth paste with baking soda, though. You still need to fight cavities.
To do both jobs, perhaps buy Arm and Hammer tooth paste.

Get a soft bristled brush so it doesn't scrape off any enamel.

Brush your tongue! The bacteria on it can still affect your teeth!

If problem persists, try whitening strips, mouthwash or see your dentist.

Hope this helps!
Source(s):
My mother was a dental hygienist, and went crazy with oral hygiene. :)