WordPress get_template_directory_uri() not returning https and only return http

Make sure the WordPress and Site Address URL under the general settings include https  The image shows, http, you need https get_tample_directory_uri() uses the site_url() , tacking on the name of your WordPress content directory, and the location of your theme plus its name. site_url will return http if is_ssl() is false, which depends on your General Settings, as stated above.

Where to start in the WordPress codex?

I think you should start here When I was back in school professor started on the loop http://codex.wordpress.org/The_Loop I did add this here too. http://codex.wordpress.org/Template_Hierarchy and do hands on project and then do searches whenever you need to find out how to do something.