By default, any product you create in GrazeCart is not charged tax. You can control whether a product is taxed on a per-product basis. This setting can be found under each product's Settings tab. This setting determines whether a product is taxed or not, it is not where you determine the tax rate.
The tax rate is set for each of your pickup locations and delivery zones. Each location and zone can have its own tax rate. Any taxable product will be charged that tax rate when ordered for that location. This is useful if you have pickup locations or zones in multiple states.
Once your products that are taxable have been set up as taxable, and you have set the tax rates for your pickup locations, the tax will be automatically calculated as part of the order totals.