how to edit my layout?

I have found the CSS but when i open index.php I cannot find the HTML?

Can someone please help me?

The HTML is generally to be found in the includes/templates folder.

Zencart does not work on HTML templates, but on PHP programming.

Pages are "constructed" on-the-fly and are made up of "bits", which includes some html constructors embedded in the PHP, database stuff, images and a whole lot more. Some pages can comprise a couple of hundred such "bits"... !

While the end result is a HTML page that a browser can recognise and display, there is (in general) no flat HTML to edit in zencart. You can edit some of the HTML construction components in the template files (and a few other places).

The layout and design of the page is controlled largely through CSS, and "toggles" or switches in the database... and of course the nature of the images and colour schemes you choose.