First, make sure you have taxes configured on your dashboard. (Items -> Taxes) You'll enter your area's tax rates here. This is also where you can tell Square to factor taxes into your price that you charge or add it to the price.

Once you have a tax rate set up, click on the "Apply to Items" button and add the items you need to charge taxes for. Any items you add will automatically be added to this tax rate.

More information can be found in this help article.