I had the experience that a CE often enough was willing to let me into engineering as a "Rookie Engineer" when I asked at the HoP desk. Same with medical. So in my opinion that is a good way to learn a job and people will know not to expect you to be a top-notch engineer/surgeon with a "rookie ID".
Security is a bit of a different matter, of course, so the best would be to join into security from the start and ask another officer to show you the ropes and let you tag along. People are mostly helpful and a lot like to teach.
Well, and as Ty said, using mentorhelp is welcome, too.