As many here have said, but I will emphasize: learn the Bash programming language. Linux Survival is a very good start, and you can just start experimenting right away in your own terminal on your own computer.

To go more in depth, you can read through the manual on your computer by typing "info bash". The Info documentation browser is a command line app. You may need to install it using your package manager ("apt-get" or "pacman" or "dnf").

In the "info" app, you can navigate with the arrow keys, pressing enter on hyperlinks, typing l (lowercase "L") works like the "back" button in a web browser, typing r (lowercase "R") works like the "forward" button. Info also lets you search the index by pressing i (lowercase "I") then entering your search in the prompt, or search the full text by pressing s and entering your search in the prompt. And q quits back to the command line prompt. "Ctrl-Z" pauses the "Info" app and drops you back into the command line, and you can resume your "info" session using the "%" (percent sign) command.

Another thing that can help is to learn about the GNU "Coreutils", this is a suite of commands usually installed into /usr/bin or /bin which provides helpful command line utilities. These are commands like cat, wc, sort, cut, ls, du, cp, ln, chmod and many others. Read through the Coreutils Info manual by typing "info coreutils".

And I will also reiterate recommendations from others: learn how to use Vim and/or Emacs. Vim has the more difficult learning curve but is extremely useful for writing scripts. Emacs is better though because it lets you split-screen with manual pages, and copy-paste commands between Man pages, "Infodoc" documents, the shell, and/or a text file, all using only keyboard commands. I think it makes it much easier to learn since everything is integrated together. Ask the Emacs community how to get started if you are interested.