I want to be able to make the first page of my site the first page of my ZenCart store.  How do I go about it, what files do I have to change?

If your store is not in the root directory (public_html/), but is in a lower level directory (where it should be) like public_html/store/, you should be able to redirect your site so that it goes directly to the lower level. It might be done from cPanel or in .htacess, or you can make an empty (or splash screen) public_html/index.html with a line <meta http-equiv="refresh" content="0; url=/store/index.php" />. That would be a lot easier than trying to move the store up to the top (root) level.