Start simple.
Having gotten to grips with implementing your designs via HTML and CSS and having made a start on PHP – you’re now ready to start building your own WP theme ;) This will initially feel like quite a challenge (which it is) – but you’ll soon be on your way, I’m sure.
If this is your very first theme build, my advice is to start with a simple design to allow yourself time to explore and experiment, to make mistakes and to learn. Plan for a small site, maybe a diary of your work on your project? – and create your content alongside a simple design for the site. Work on a representative prototype (HTML/CSS/JS) and proceed to the theme build.
Content out
As with any site design, you will start with your content, then the design – followed by the theme build. When designing for any CMS, it is important to work with given content. Attempting to design a theme or template for an empty site will only result in lots of time wasted, confusions during content input and your learning process will be much slower.
Content precedes design. Design in the absence of content is not design, it’s decoration.
Jeffrey Zeldman
Even once you are confident in your theme building skills, you will still approach the work as usual: content out and mobile first :)