Before Add tax to a product, you need to add Tax. To add tax click Tax Setting under the Tax menu. Here you can three types of tax, just write tax name and click outside of the input field it will automatically save Tax name.
Now Time to add a tax on any product, Select Tax product Service under Tax button. Select product name, Tax name and tax percentage and save it.
To See how many products added under the tax click on "manage tax" button under the tax button.