Employers will almost exclusively hire transcriptionists who have completed some kind of formal training in the area or who have experience. There are many ways to obtain such training, but for most a community college or vocational school will be the best option, especially if the training includes on-the-job experience. Do not worry about whether or not they offer the opportunity to earn the Certified Medical Transcriptionist designation. This is not a required certification in general, and in fact, most transcriptionists need a couple of years' experience before they can pass the test to gain this designation. I am not a CMT, but I was able to find a job.
Courses typically take several months to two years to complete, depending on the school chosen. Many vocational schools include the cost of books and course materials in their tuition, which may be helpful after you have landed a job. If you already work in a medical field, and simply want to switch to working from home, you may only need to learn transcription, as you may already have the vocabulary you need, but think carefully, as a refresher course may not be an entirely bad thing either.
It is also possible to take transcription courses at home. This option is more flexible but may be more challenging in other ways. Your selection of a course of study should depend on how you will best learn what you need to know.
You will need an assortment of reference books. Some you may recieve with your training, but you may find you need others. See the equipment page for suggestions.
College Courses
College courses are best for most people. Having an instructor to help you through the difficult parts and to keep you working on a schedule can be a great benefit. Many schools also offer job placement assistance, which may not be available through home study courses.
Community Colleges
Perhaps the cheapest method of instructor-led training you can get, but it may not be quick. A quick look at a local community shows a 22-25 unit requirement for certification in Medical Transcription. This would require a minimum of two semesters' work or more, depending on prerequisites and whether you attend part- or full-time.
Costs vary by what state you live in. Check with your local school.
Vocational Schools
Vocational schools cost more than community colleges, typically into the thousands, but may offer more flexiblity when it comes to the timing of your class. This may not be a quick solution either. With a quick look at a local vocational school's webpage, I found that its Medical Transcription training program takes about 28 weeks of classroom time and four weeks of an externship. Check to see if textbooks and other supplies are included in the cost.
Home Study
Home study courses offer the most flexiblity in many ways, but they offer challenges as well. You must be capable of working on your own. On the other hand, if you want to work at home, that's a skill you need. You'll need special equipment for this option in many cases, such as a foot pedal.
In the time I've spent researching medical transcription schools, three names keep coming up as the ones that employers actually hire from regularly: Career Step, M-Tec, and Andrews School of Medical Transcription. Of them, CareerStep is the most affordable. The others have a still better repuation with employers, but CareerStep graduates are plenty well enough trained to find employment. My former employer, Medquist, requires that you take the Gold level or above from CareerStep (last I heard, anyhow), which is my recommendation as well. Expect to take a minimum of 6 months, often 9 months or more, to complete a course.
Which option is best? That depends in large part on how you best learn and what is available in your area. There's no one definite right answer that will meet everyone's needs.