ZenCart Tutorial – How to configure sales tax in ZenCart
Open your administrative interface and point to the Locations/Taxes menu. Select the Zones Definitions option. From the menu you can double click to edit an existing zone or create a new one. On the image below is show how to edit an existing zone. Enter a description, according to your preference.

Click Update.
After the zone has been created, please click on the Details button to the right.

Click Insert. Select country and zone. Click Insert.

Select the Tax Rates option from the Locations/Taxes menu.

Click the New Tax Rate and select your tax zone. Set the tax rate and enter a description. Click the Update button at the bottom to apply the tax.

In order to make the addition of new products quicker, you can select a default Tax Class, to be applied every time you create a new product. To do that please go to the Product types option of the Catalog menu.

Select the product type, click the Edit Layout button…

and enter the tax ID number, that applies for this type of products.

Please, feel free to contact us if you have any questions or recommendations about this tutorial
Source: http://www.siteground.com/tutorials/zencart/
This tutorial is provided free of charge and without any warranty of any kind.