I no longer maintain a tutorial for Markdown on my website. To review all of the syntax of the original version, go to Daring Fireball, the website of John Gruber, the author of Markdown, at daringfireball.net/projects/markdown/syntax. The original version is written in Perl.
You should be aware that there are also many other implementations of Markdown available. Some of these significantly extend Markdown to include markup for advanced features, such as tables and footnotes. There are also internal formatting differences between some of these extensions and versions that are translated to a programming language different from the original Perl.
Because of the variation in how these other implementations interpret markup and because Markdown itself has some formatting ambiguities, there is a project to produce a "common denominator" version of Markdown. The project is called CommonMark. Read about CommonMark.
You can check the differences in how the markup is rendered at the Babelmark page. Type your markup in the box and Babelmark will show you how all the major implementations interpret the markup.