what is the best way to learn motion graphic for beginner?

If you want motion graphics and animation guidance that will help you be good at it, buy this book: The Illusion of Life: Disney Animation. 

I have it. I've had it for about 26 years. I read it about twice a year to remind myself of the basic design principles.

To learn After Effects, spend about an hour exploring the Learn Workspace or the tutorials on the Home Screen for After Effects. Then, start in on the User Guide.  It will take you twenty or thirty hours to get familiar enough with the UI to figure out when a tutorial you are watching misses something in the explanation. Enthusiasts prepare many of the tutorials found by searching YouTube and are nothing more than a rehashing of a recipe they copied from somebody else. Many times, the explanations and even the workflows are pretty poor. If you have problems with a tutorial you found and can't contact the author, post a link to the tutorial when you ask questions on the forum.

The difference between graphics that just move and graphics that tell a story well is in the planning, timing, and execution of the design. Most social media platforms have tools that will automate text and graphics. They seem very popular, but you'll never get paid well for that kind of approach to motion graphics. You need to understand how the eye reacts to motion, color, and sound (Frank and Ollie's book) and a lot of practice to turn Motion Graphics Design into a career. 

It's also a good idea to start a collection of links to samples that resonate well with you, your family, and your friends. I look at sites like https://motionographer.com/ and https://www.artofthetitle.com/ and see what resonates with you. 

I hope this helps you get started. AI can make graphics and words move with no real effort on your part. If you are going to be successful, you have to put your own spin on your work and give it something that AE can not achieve.