What professional help should I seek if depressed?

Encourage. Psychological or psychiatric evaluation. Comprehensive assessment allows for diagnosis which leads to a good treatment plan.
Depression. Depression can be a very troubling thing for anyone to cope with. A good support system is always helpful (friends, family, etc). Professional help is always important, especially if you have thoughts or feelings of wanting to hurt yourself or others. If you have these feelings, you should get help immediately - day or night. If nothing else, go to the closest emergency department. Good luck.
See below. First see your primary care doctor as soon as possible, especially if you have suicidal thoughts (even just occasionally). You may be referred to a psychiatrist and/or psychologist. Meds help but they are rarely ever successful without therapy.