I don't have much info on getting "certified" to be an MT. I got with a woman who trained me for 10 weeks, 2 nights per week - she had her own half million dollar MT business for 30 years. So I sorta got into this through a "back door". It is highly recommended to take a good extensive course through a local college or through a reputable MT school like The Andrews School (you can find info for this at mtdesk.com).
The pay is excellent. I have yet to find something else I could do at home, no weekends, part-time (25 hours per week, M-F only) and I make $50,000/year. Not all MTs make that much and some people make even more... I've known MTs full time in the six-figure income but it is few and far between (you should see the on-line arguments as most MTs think the pay sucks but they aren't trying hard enough in my opinion), but it takes lots of practice, being on your own (not working for anyone else who takes part of your pay), and cutting all corners (like utilizing short cut programs - I use the heck out of ShortHand - and doing all clinic notes for the same set of doctors over and over and over again).
I highly recommend it but I always make sure people know - yes, you need to put money into it to get anything out of it, and you need to be highly detail oriented and determined to really make something of it.
Okay, I'm getting long-winded... must shutup now. I'm sorta just bored of it right now and want to expand my horizons!