I strongly recommend you do your research. There are dozens and dozens of schools to choose from - so make sure you find the one that will fit your needs best.
A good place to start is www.ahdionline.org. That is the professional association that represents medical transcriptionists. They have a very stringent approval process for schools offering transcription. All of their approved programs are solid - but you will even find drastic differences between those on the list! On average, schools take 9 to 18 months to complete (if a school tells you they can have you employable in 3 months, run!). Some have devoted instructors that will work with you every step of the way and answer questions for you. Others have "graders" who only grade your reports but are not available otherwise. Some have technology that grades your reports and you have no instructional support at all. So ask the right questions.
DISCLAIMER: I work for MTEC - a school that offers medical transcription. I'm obviously going to be biased toward my school.
Best of luck to you!