What position are you starting from? My old workplace was a university group where we (admins) were recruted from the available pool of PhD students. So I'm used to guiding people from "no knowledge" to "enough knowledge to be dangerous". The first step was to force the prospective admins to run a specific system on their "productive machine" and keep it in such conditions that _everything_ works.

This way, a complete admin newbie would learn about digging through the systems by working out the kinks of practical everyday problems. Remember, this is only the most basic instruction, nowhere near enterprise-grade.

If there was a "prospective admin" who had never before run Linux, I'd tell them to install and use Ubuntu/Mint. (Those guys whould usually only be trained to be a helping hand for a "senior" admin.)

If he'd already used Ubuntu at home, I'd tell them to start using Debian and work out how to set up an SSH server and set up their home machine so they could access it remotely.

If they had dabbled with Debian, Fedora, SuSe or something similar, I'd tell them to install Arch and set up some "interesting things", like a mail server or a nis server.

If they were using Arch or Gentoo at home, I'd just personally show them the important things about our system and have them wingman with me for a few days.

If you are already an advanced Linux or BSD user, my approach is of course not appropriate. Instead I'd recommend to pick skills that you want to learn (iptables? Exim?) and set that up. Read Manuals! Read RFCs!

Best of luck.