The best way to whiten teeth for free?
DO NOT PUT BLEACH iN YOUR MOUTH!! My wife uses the strips but my dentist says they weaken your enamel leading to future problems. A decision you will have to weigh.
Brush them :)
There is no free way. I'm pretty sure putting bleach in your mouth will kill you. Other than that, I recommend whitening strips for about $50.
Do not drink liquids that will stain them and/or brush immediately after drinking/eating!!
Add baking soda to your toothpaste, use everytime you brush you teeth.
You can buy toothpaste with baking soda already in it, so you don't have to add it. Brush them at least 2-3 times a day