A Practical Guide to Fedora and Red Hat Enterprise Linux (7th Edition) https://www.amazon.com/dp/0133477436/ref=cm_sw_r_cp_apa_3Qy8ybA04XHJJ
First step is to get yourself familiar with the different types of operating systems and if you are going to be doing any sort of linux field you need to be familiar with RedHat based operating systems. I recommend CentOS or Fedora for that.
https://www.virtualbox.org/wiki/Downloads The link above is for a pretty easy to use virtualization software that is free to use.
If you are interested in linux the book above is really good, just read through the chapters, do all of the exercises and you will learn more than you could have from any class. The chapters will guide you in the right direction, but the exercises really test your ability to tinker and play with the command line, read the man pages and figure out how to get the most out of each command.
I'm currently in the Linux Admin Field, I don't have any certifications and went to a two year college(which honestly I regret). Just had a passion for linux and learned using books and just playing with virtual machines.
I can't say much about how to learn on the Microsoft side, but I can tell you that linux is the way to go. Once you figure out the basics, going back to administrating microsoft servers is going to feel so outdated.