To add a tax rate to a product, follow these steps:
Go to Items & Orders > Products.
Select the product/s you want to add a tax rate to using the checkbox.
Click on the dropdown box.
Select Update tax(es).
Select the tax rate you want to apply to the product from the dropdown box.
Click Save.
Note that by default, a tax rate is added to your account when you are first set up. You can add additional taxes and organize them into tax groups which can be applied to your sales.