The following sections are introductions to the basic principles of the language of Mathics3. A few examples and functions are presented. Only their most common usages are listed; for a full description of a Symbol's possible arguments, options, etc., see its entry in the Reference of Built-in Symbols.
If you Google for “Mathematica Tutorials”, you will find dozens of other tutorials.
I (Rocky Bernstein) like "Power Programming with the Mathematica®: the Kernel" by David Wagner. Even though this is old, and the book covers Mathematica® 3, it has a lot of useful information that is still relevant. A PDF of this is available here. Wolfram's "The Mathematica® Book, Fifth Edition" is useful. If you google for this book, you can probably find a PDF of it for free.
Online, there is An Elementary Introduction to the Wolfram Language. In the docker image that we supply, you can load “workspaces” containing the examples described in the chapters of this introduction.
Be warned, though, that Mathics3 does not yet offer the full range of features and capabilities of Mathematica®.