Is it safe to use baking soda to brush my teeth in order to make them whiter? How often can I do this so it wont harm my teeth?

Inefficient. Does little to whiten teeth. If darkening due to surface stain a professional cleaning by your Dentist followed by using a whitening toothpaste may be all you need. If darkening due to internal causes, professional whitening by your Dentist is indicated. Save the baking soda for use in recipes. Though it won't harm teeth, it really will not help either.
Teeth whitening. You can use over the counter products and baking soda. However, they are not so effective and/or stable. OTC bleach and whitening tooth paste could be used as a supplement only. A professional teeth whitening without laser, followed by home whitening system with custom trays for the long term effect. You will be very happy with the results.