With the start of First Access, a host of new tutorials has become available. Some older videos are being taken offline due to their outdated contents.
The APEX Handbook
The Handbook contains both written step-by-step tutorials and a wiki section for looking up information. Read tutorials 1 & 2 (2.1-2.5) to get started. Tutorials 3-7 cover more advanced topics. They all correspond to the video tutorials with the same titles.
The video tutorials
All introductory tutorials and all advanced tutorials are also available in video format.