Brush. Beside the whitening make sure brush at least two times a day and also brush the roof of your mouth and your tongue.
First see dentist. First, you should see a dentist to make sure your teeth and gums are clean and healthy and all necessary dental treatment has been completed. Then you can discuss the many whitening methods available to you. Assuming you have no medical issues, routine and thorough tooth brushing, flossing, the use of a tongue scraper and possibly the use of listerine or crest prohealth. Consult with dentist.
Peroxide. Peroxide gels are very effective in whitening teeth. Rule out gum disease if your moth is not "fresh".