My sister and cousin are both medical transcriptionists, and my cousin learned it by working at the hospital. My sister went to a community college and took a one year course on it. I took some classes as well and it is not as easy as it may seem. A lot of dr's have accents, and you need to be a very good speller.
My suggestion for you or for any career, is to talk to the place where you would like to get a job (hospital, dr's office...) and ask them what they would require from you. Some places require you to have a special accredidation. I know that my sister has told me of countless people that have applied at the hospital who took courses at home that really are not prepared for the "real world" experience.
Good luck. My sister loves it. She works graves at the hospital and supports her husband going to school and has 6 children at home right now. It is just temporary for her, but she enjoys it.