1- Guides already on Linode.com on DigitalOcean.com
2- Official documentation
3- Set up a command line virtual machine or any command line tool as a basic lab
4- Will to experience and test and build.
5- online free videos.
So many guys on youtube.
I like videos bc the way things on linux are done can be done in different ways with different tools.
I like those videos that explaining everthing, and I don't like those which just tell you about some lazy scripts from gits. Most of those lazy scripts don't work. And I stay away from Indian youtubers. Very bad English, very bad explaining, very bad content, very poor explanations. I just can't forget those brown hands soldering cellphone camera module to a USB cable and snap to PC and work. They are all the same.
I just remember "HackerSploit" since he has an accent and I like the way he speaks English. Very easy to understand. And his channel is very old and updated frequently. He's worth donation. However his old YT vids, the font size of commandline is small. I always change command line to bigger size for demonstrations.
There are many youtube linux wizards out there. I find "ChrisTitusTech" has great content for the "linux experience" itself as he mentions it. The list will be updated.
I would stay away from Nullbyte. The linux security guide had a config mod to prevent rubber ducky attack, which locked my keyboard out. and then had me lose an installation. Stay away from them since they have lots of bells and whistles, but they don't deliver it.