tutorial

By far the simplest and easiest way to get to know any of the Adobe Suite products is to start with the Adobe Getting Started tutorials. You'll find the After Effects section begins here: https://www.adobe.com/learn/after-effects?locale=en&learnIn=1

After you have a handle on the basics and have looked at the training offered by any third-party effects suppliers like Video Copilot, Maxon, Boris Effects, or others, you may want to dig into some specialized paid online training. The best courses depend entirely on what you want to create using After Effects. I know many effects professionals who only concentrate on a few of the thousands of different types of projects you can create using After Effects, so they are known as experts in Compositing, Motion Graphics, or even Rotoscope instead of trying to be experts in everything.   When you pick a specialty you want to concentrate on, let us know what it is so we can point you to some good information.

You should also be aware that many of the 'tutorial videos' you find on YouTube are prepared by amateurs, so that the workflows may be inefficient. Make sure you vet your trainers. Some workflows that work for specific shots will not work at all on a shot from a different angle.