You can start here - https://docs.moodle.org/dev/Main_Page
Rather than reading docs, I prefer to implement something from scratch that leads for learning by doing.
For example, for the BOOST theme you can start by modifying CSS at http://yourmoodlesite.com/admin/settings.php?section=themesettingboost.
Or you can start to develop a plugin from scratch. You can check the source code of the existing plugins for guidance.
And feel free to ask in this Development forum whenever you stuck or have questions.