I am trying to consolidate links for the Helix approach that Sitecore folks have created:
http://helix.sitecore.net/index.html (You should definitely start here!)
http://helix.sitecore.net/introduction/index.htm
http://sitecoreblog.evident.nl/posts/2017/02/20/helix-in-practice-
https://sitecorebasics.wordpress.com/2017/02/17/sitecore-helix-basics/
https://github.com/Sitecore/sitecore.demo
http://habitat.demo.sitecore.net/ (Habitat demo site)
https://github.com/Sitecore/Habitat (Habitat Git)
Feel free to add your links in the comments section.