It is one of the major questions for any developer in any technology. To start development in any technology, first of all, you should go to official website and start exploring. Go to documentation and dive in.
Steps for learning:
- Learn HTML
- Learn CSS
- Learn PHP
- Learn WordPress from https://codex.wordpress.org and https://developer.wordpress.org just dashboard.
- Start understanding the hierarchy of WordPress and file structure.
- After all WordPress Building Themes from Scratch Using Underscores.