Qualifications
You don't necessarily have to have a degree or training in medical transcription to do the job...however, if you don't, you'd better have a close relative who will hire you. Many community colleges will offer degrees and certifications for registered/certified health care documentation specialists. These qualifications will help you get and keep solid work and make a decent wage (source).
You need to understand medical jargon, have excellent spelling and grammar knowledge, solid keyboarding skills, and the tenacity to get the job done.
You'll have to be computer savvy and be able to decipher what people are saying in a recording when they mumble, have heavy accents, or speak way too softly.
The job also requires an understanding of basic human anatomy, drugs, and diseases (source). You should also be sharp in the subject you're transcribing. If you are transcribing mental health information, it would be good to know the difference between mania and manic; schizophrenia and schadenfreudism.