Where can i find the files that let me customize my template?

The way I've set up my standard site is through includes, where i'll have a index.php page that includes the header, menu, and footer. I'm assuming that's kinda how zen cart is set up.

What I'm asking is where is the original file that includes the rest?

I assumed it would be zen/index.php (where zen/ is the install directory) but it doesn't even have the html and head tags

things I'd like to accomplish:
add an additional style sheet,
change the layout of certain sections,
and be able to isolate included sections to have an easier time figuring them out.

I wasn't sure if this should go in the layout section or this.

as per the posting tips,
i have version 1.3.8, not upgraded, no add ons, one template

if you could get me pointed in the right direction, i'd really appreciate it,

thanks,

Zencart is a database driven dynamic PHP program which builds pages 'on the fly' from various components. It does, as you mentioned, have a header, center column and footer, as well as two side columns.

The easiest way to figure out what is where is by installing the Firefox browser and the Web Developer plugin for Firefox. You can use those to see where in the stylesheet you change the styling of the various page elements.

Ctrl-Shift-Y will allow you to hover over a section of the page and see the class and id names for that section in the box at the top. The statements in the stylesheet that begin with # are id's and the ones that begin with dots are classes.

Ctrl-Shift-E will open the CSS editor and allow you to edit the stylesheet and see the changes in real time without changing anything permanently. A very handy tool.

These may help, too:

https://www.zen-cart.com/tutorials/index.php?article=38
https://www.zen-cart.com/tutorials/index.php?article=39

/includes/templates/your_template/common/tpl_main_page.php is the file that brings in the major sections of the page.

Most of the things you mention will be accomplished not by editing this file, but by admin settings and other control features built into Zen Cart.

Lotsa tutorials available: https://www.zen-cart.com/tutorials/index.php?article=142 and related links
Reference on adding more stylesheets: http://www.zen-cart.com/wiki/index.php/Customisation_-_Templates_-_Stylesheets

There are free templates available for Zen Cart which you can use as a starting point or as a guideline to creating your own template. You can see them in action at http://(sorry, site offline)/. Use the pulldown menu to change templates.