ROS2 help you develop a robot faster and provide a structure for development. That way, the software developed is maintainable and we can reuse package made by another person.
TO DO
If you want to learn how to develop using ROS2, you must first learn the basis of programmation. ROS2 is using Python 3 and C++.
Every tutorial in this list can be done using docker. If you never use docker before, I suggest that you read the docker tutorial first. All the tutorial listed are free.
1 - Official tutorial : In this tutorial, you will learn how to use the CLI for ROS2 and how to create a package. To understand most concept in the club, you only need to do the two first section (Beginner : CLI tools and Beginner : Client libraries)
2 - DotBuilder ROS2 playlist : This YouTube playlist will help learn you to learn the fundamentals of ROS2. It's basically the official tutorial in a video with some add ons. The version used is foxy, you just have to replace this word every time by humble and everything should work.
3 - The construct ROS2 playlist : The construct is a well establish ROS2 school. They offer great courses on their site and some are free on youtube. This is almost the same tutorial as #2 but if you know ROS 1, they explain the difference better to help you understand how ROS2 works.
4 - Robotogeddon playlist : This tutorial focuses more on ROS2 with C++.