Creating my own personal theme: help

you need an index.php file

review https://codex.www.ads-software.com/Theme_Development

Does this mean I have to use PHP to code my wordpress site?

yes – and HTML

So,

I’ve on a header and nav bar so far…

Is it too late to simply rename the file as index.php (instead of index.html) and incorporate php elements?

Nope; in fact, I recommend doing that as soon as possible. In addition to what alchymyth suggested, I’d also recommend reading Template Hierarchy, to get an idea of how WordPress puts together your website.

Okay,

Great! And what about the stylesheet.css file, should I rename that, too?

Thanks for the help.

you will need to have a style.css in your theme, which should start with a structure like this: https://codex.www.ads-software.com/Theme_Development#Theme_Stylesheet

you could rename your stylesheet.css to style.css and add the necessary data to the top if you haven’t already.