My start with the command line was the huge unix book by O'Reilly with that huge-eyed maki on the cover.

I learned most of the commands by simply reading it from beginning to start and trying the commands.
This is possible in OSX, too, I think.

But like the others said, after some time a free OS is better to gain a deeper understanding.

I am currently using Arch, but I would say that something without systemD (like Void Linux) may be easier to understand (runit
seems pretty cool).

Have fun! See you on the forums!