First, the most important thing you need to know is that you do not need medical transcriptionist certification in order to work at home as an MT. In fact, most doctors still do not even know that certification exists for the medical transcriptionist! While there are some MTs who have certification, the majority do not.
Second, you may be wondering exactly what is a Certified Medical Transcriptionist (CMT). It is a voluntary certification offered by the Association for Healthcare Documentation Integrity (AHDI) [They were formerly called the American Association for Medical Transcription (AAMT) and many people still use this name, though technically it is not correct].
AHDI's purpose for trying to encourage all medical transcriptionists to be credentialed is to encourage high professional and ethical standards in the medical transcription industry. They felt that if there were certification it would be a sign both to the public and to doctors that the transcriptionists who obtained it had worked and studied hard, really knew what transcribing reports is all about, and had several years of experience to prove it!
If you decide to become a Certified Medical Transcriptionist you will need to take a test given by AHDI. Because they want to be sure that you are truly ready to take and pass the examination the first time you take it. Those two years of experience should include multiple dictators that speak English as a second language, many different report types including surgery, and some specialty clinic work as well. If you have all that under your belt you will have a much higher chance of passing the examination the first time.
While you don't need to become certified, some MTs find it brings greater financial reward, while others report no change at all in their financial picture. Some employers will grant pay increases to those MTs that have the CMT designation while others will not. It would be worth your while to read through the material on AHDI's web site to decide if this is right for you to pursue in the future when you have the suggested experience. They suggest two years of acute care (or equivalent) experience before you are allowed to take the CMT test, so you don't need to focus on this too much right now if you are just starting out in this career. You just need to know it exists and it's something to aim for in the future if it interests you.
It's not difficult to obtain medical transcriptionist certification, but it does require experience and diligent study time so that you are well prepared. In the meantime, though, not having certification will make no difference to you being able to embark on this new career path.