Rod has given you some good pointers. While a knowledge of HTML will be useful, you need really to get your head around PHP code. There are no "HTML PAGES" in Zencart. As Rod says, the eventual HTML that is delvered to a person's browser screen is built "on-the-fly" by the PHP. The "pages" do not exit until a visitor calls for them:
EG: When a visitor types in your site URL, they a "calling" for the landing page.
The PHP software says:
"Hey... a new visitor! I'd better note down this fact (so it sets a session ID applicable to that user) and I must then assemble the "home page" for them.
The PHP then gathers up all the components needed to display a "home page". These will include IMAGES, product data, welcome message, header and footer info... It CREATES A BRAND NEW HTML FILE and then it sends this off to the visitor's browser screen.
When the visitor clicks on a link, that "home page " EVAPORATES FOREVER. It completely disappears. It is "thrown away"...
... and a BRAND NEW page is compiled by the PHP, where the contents of the next "page" are determined by what the visitor has clicked on.
Let's say they click on a product that is featured on the home page.
The PHP says: "OK... HOW do they want this particular data displayed? I'd better fetch the NAME of the product, the DESCRIPTION of the product, the PRICE of the product, the PICTURE(S) of the product - and all the other components (there can be several HUNDRED on a single page") and it then ASSEMBLES that data according to the WIRE FRAME (the template)... and it "parses" the data into a HTML file, which it then delivers to the browser screen.
This happens in a matter of milli-seconds.
Then, the visitor may click the ADD TO CART BUTTON...
The PHP gets the instruction, and says: "HEY... he want to add this to the cart! I must now temporarily write this data to the database, and go about doing some important calculations (cart total, weight total...) AND I must now build a NEW PAGE showing the visitor what they have in their cart."
The previous "product page" EVAPORATES.
A NEW PAGE is built, showing the cart contents... Along the way, some background CALCULATIONS have been done as well, and some temporary data (relating to that session) has been written to the database. (IE: the contents of THAT PARTICULAR VISITOR'S CART).
AND SO ON IT GOES.
The TEMPLATE can be looked at as a WIRE FRAME, where certain TYPES of data are configured to appear in certain AREAS of the screen.
As I said, unless you are VERY SKILLED AT PHP, CSS and MYSQL - and you know zencart inside-out - building your own template will be very, very challenging.
Your current knowledge of HTML will not help you at all (other than that it makes you appreciate the protocols of a web-page).
SO... Just use one of the existing free templates, and alter the graphics, the colour scheme and some of the easier CSS commands.
START SELLING... Which is why you want a eComm website... Worry about "what shade of turquoise" to use later on.
The FOLLOWING TWO SITES use the SAME BASIC TEMPLATE (yes... the SAME TEMPLATE). All we have done is a few minor alterations.
officefrog . co . uk
callmecupcakes . co . uk