My mouth tastes bitter. what should i do?

Many causes: Have you had this sensation for awhile or is it recent? It could be side effects of meds or supplements, change in foods, change of toothpastes or some underlying dental/gum problem. You should brush your tongue when brushing your teeth. I would try and keep track of when this taste occurs and address.

Several: Brush your teeth and tongue? Large cavities? Gastric reflux? Post nasal drip/ sinus problems? Dissimilar metals (fillings) in your mouth?

See a dentist: You either have gingivitis, with pus oozing into your mouth, or a brain tumor which a neurologist can find. Odds are it's the gum disease....