So, a psychiatrist is an MD and, as such, can prescribe medicines. The psychologist is-- to all intents and purposes-- a therapist. (I'm painting with super broad brushes here, there are psychiatrists that operate as therapists first and medicine prescribers second, etc.) I suggested a clinical psychologist because they're usually the ones who have the training to act as a therapist while also understanding how the brain works. They're often the people trained to assess someone for ADHD.

No matter what, any good doctor will want to have you assessed by a psychologist or a psychiatrist. You may want to ask your GP if he or she is willing to prescribe the medicine provided a psychologist/psychiatrist diagnoses you. That's usually the easiest thing to do.