Welcome to the forum, and good luck with your new online venture!
The forum is full of useful information and you will find that there is plenty of help available, through the FAQ/TUTORIALS and WIKI, as well as postings on the forum itself.
Before you concern yourself with customisations, I'd like to suggest that you create your own CUSTOM template folder system first.
The reason for this is to preserve your changes when you make edits to standard template files. Later on, when you UPGRADE, your customised files will be preserved, allowing you to upgrade your ZC software without the risk of over-writing any changes you have made.
You are well advised to follow the CUSTOM template protocols whenever you consider making a core-code change to your site. Most of the time, these changes will be to TEMPLATE, LANGUAGE and CSS files, though occasionally you may wish to make some adjustments to other files.
To start you off on the process of CUSTOMISATION, read this tutorial first:-
https://www.zen-cart.com/tutorials/index.php?article=142
You will create and administer your CUSTOM template largely through your FTP client, or via your host C-PANEL interface. This is because you will be working with directory folders and files within your website root directory.
After you have read and enacted the steps in the above tutorial, you are well advised to spend some time looking through the ZC directories on your server - particularly those in includes/templates and includes/languages...
... wherever you see a folder titled "classic", you should create another folder at that level, named with your CUSTOM template name.
In this way, your sit3e will be physically structured for editing files and placing them in your CUSTOM folder(s).
The process of populating your CUSTOM folders is called OVER-RIDES, and you will often see references in the forum to the OVER-RIDES system.
It's also advisable to read the FAQ for more clarity on HOW the over-rides system works, and the functional purpose of your CUSTOM template set-up.