Depends what you’re trying to learn, and how much of a beginner you are. If you want to learn the shell, try the Software Carpentry tutorials:

• The Unix Shell - Software Carpentry
• Version Control with Git

If you know the basics, you might try honing your skills with CLI Mystery (murder mystery puzzle).

You’ll probably want to learn how to use the following:

• SSH
• Command-line text editor. Choose one of the following:
  • Nano <- Smallest learning curve
  • Vim (or Neovim) <- My favourite (See Mastering the Vim Language)
  • Emacs <- Some people like it

The final tip is: It’s usually better in the long run to spend 2 hours reading the documentation than 2 minutes searching the web. Reading the documentation helps you to understand the big picture, and gives you a much better foundation. Of course, if you’re reading the documentation and don’t understand something, searching the web is an OK way to figure it out.