There are a lot of functions you can turn on or off using the admin side of your cart. The "e-start Your Web Store with Zen Cart" book is a user's manual for admin features (link on home page).
The override system is for customizing the looks and text of your custom store pages.
These tutorials will help you get started. You will be working with copies of the orginal (core) files. You'll need a good text editor, like notepad++, and an good FTP program (Filezilla is recommended frequently). Download Zen Cart and unzip it on your hard drive to learn ZC's file structure, and to make file copies.
https://www.zen-cart.com/tutorials/i...hp?article=143
https://www.zen-cart.com/tutorials/i...hp?article=142
Hope this helps!