By default installation two plugins are available to help you with setting a tax. You may have more than those, if your country require a different options than available here.
Available plugins offer a fixed rate or by area.
Fixed rate is quite simple plugin. Offering a different rate per category. If your country has just only one rate per all items, just simply will have just one record with a rate. If your country offers a different tax percentage depending of the product group, need to add categories with a different rate of tax. Simply at it is.
Tax by area is more complex, as you need to define area which may be country, state or even zip. Even if this is more complex give you more options.
That option allows to create a name for tax grouping products in your store. In other words, if your country offers different tax per some products category, you may apply those here.
Define settings of the taxes. Here you may define the way of showing the taxes to your customers (by selecting if prices include or exclude the tax), define the way of showing tax prefix, if shipping or payment fees are taxable. Changes here changing the way of the prices and totals of the orders.