How to start developing my own WordPress theme?

Hi there,
I am using WordPress from 2.5 years and I have developed over 80 websites using WP for my clients. I’ve used many premium and free themes for development. But Now, I want to develop my own theme from scratch. I have ‘medium’ level in CSS, HTML & PHP. Although, I have modified many themes and plugins in my way but I am being confused to develop new themes. Anyone here to help me about these? Is there anything else to learn? Forgive my bad English 😉