What is the best way to learn shell scripting in an easy way?

When you say "easy way" that is highly subjective, something easy for you may not be easy for me and vice versa.
There's tons of materials on youtube and others that you can learn from. What have you tried already that was not "easy"?
And remember, only practice will make you better at it...

If you have a traditional sysvinit boot, study the startup scripts. List a script at one terminal, try out individual steps at another to see what they actually do (but don't do that as root in case you damage something) and use a third terminal for calling up man pages.

Sitting here and seeing what problems arise and the solutions suggested is a great resource. Over time you will be able to solve them and contribute.

Time and practice.

yes,
1. practice
3. practice
4. practice
.....

Do you have any previous scripting or coding experience? If so, you might be able to get away with a reference guide or small tutorials.

This is a good Bash starter tutorial: https://www.youtube.com/watch?v=oxuRxtrO2Ag (Joe Collins - Beginner's Guide to the Bash Terminal)

There are also tons of tutorials on bash and scripting that can be found with a search such as "bash tutorial".

While youtube has its place, the written tutorials are still quite relevant and more in-depth.

Why do you want to learn shell scripting?

Give your other thread, the answer might be: by reading the websites/books/etc recommended by your teacher.

If you have such recommendations but do not find them easily understandable, identify them and explain what you find challenging. That way, people can recommend solutions without the same shortfalls and/or help you past those initial hurdles.

Well, if you want to learn "shell scripting," maybe the very-first thing that you need to learn about is: #!shebang.

If the very-first line in your "shell script" file looks something like #!/usr/bin/php (for example ...), then the shell will immediately launch an instance of the specified "programming-language interpreter," and hand the rest of the contents of the file over to it. "And you will be none the wiser."

And so, what this means is that "you are not-at-all limited to 'shell scripting.'" Any full-featured programming language that is installed on your computer – and usually, there are several – can be used to carry out a "command."

The only "shell" that I am aware of which ever strived to incorporate "a programmingly-serious built-in scripting language" into itself was the Korn shell ... ksh. But it never really gained much traction. The "shebang!" principle was much more elegant. And, if you take a look around, you'll find that many of the "commands" that you use every day are implemented in this way.

The paltry "built-in" scripting capabilities of "the ubiquitous shell" are, most fortunately, irrelevant.