All medic promotions are from within, you have to be on the job for at least a year (BLS) prior to taking the promotional exam. The exam is competitive, written, practical and oral boards, top 6-10 get into the internship. Internship consists of a month of clinicals in hospital, then 9 weeks in the field, if all goes well an exit interview follows and then a 6 month probationary period.
You be a Mass medic at time of test, the department has on occasion run its own in-house medic program, the current one is scheduled to finish in another year or so, I have my doubts about the department running another one any time soon. Good luck!