Hi Stephalinamay,
I would say one of the most important things a new MT should know before starting his/her training is proper grammar. I've seen MTs who have finished school and know their terminology quite well yet have run-on sentences, improper punctuation, and do not use nonmedical words correctly (sight, site, and cite come to mind). If you have grammar and punctuation down before you start your training you will definitely have an advantage and you won't have to learn grammar and punctuation at the same time as learning medical terminology.
How do you know if you need to brush up?? Well, there are lots of grammar tests on line--take some and see where you need work. Pick up a grammar book like the Gregg Reference Manual (there are lots of other good ones too--you should be able to buy a used one on eBay for around $12) and study it.
Just my opinion, of course--hopefully others will post with more great ideas!
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kkb, who considers herself a newbie after 18 years and still remembers what it was like to be a newbie!