Welcome to the module developer's guide for Drupal 8!
This section includes tutorials and other information that you need to create custom modules for Drupal 8.x version.
An excellent overview: Understanding Drupal 8
Other general resources
There's an excellent presentation on working with the Drupal 8 API, Los Angeles 2015: Drupal 8: The Crash Course (May, 2015).
There are updated slides available here from the MidWest PHP conference. (March 14th, 2015)
There is also a Sitepoint series on developing Drupal 8 modules that you can check out.
Note that core Drupal has moved on since this tutorial was written, and the code no longer works without error. You can find some explanation of the changes needed, as well as links to the updated code here.