Psychology student here, just about to finish my Masters. Basically we diagnose what may be wrong through trial and a series of tests, speaking to the patient, receiving feedback etc. We then refer them to a GP of their choice and contact said GP with the information we have gathered. They then prescribe an appropriate medication, like fluvoxamine for example. Thing is, antidepressants for example are simply used as an complimentary asset for the real treatment, which is therapy and management based for the most part. You don't simply give someone pills and send them on their way.