To change the title of your site, go to includes/languages/english/meta_tags.php

Find:

// page title
define('TITLE', 'Zen Cart!');

// Site Tagline
define('SITE_TAGLINE', 'The Art of E-commerce');

Change the text highlighted in red.

and save to incudes/languages/english/YOUR_TEMPLATE/meta_tags.php

Here is a tutorial on setting up taxes:

https://www.zen-cart.com/tutorials/index.php?article=76