First go to your GP or PCP and tell them what's going on. They'll run some tests and then refer you to a psychiatrist.

The psychiatrist makes diagnosis and treats with medication. Usually an 1-2 hrs the first time then 15 min. every 1-6 months after that.

You may want to see a therapist to help you learn coping skills and track your mood swings. If they share notes with your psychiatrist you'll have better care, usually.