Hi,
In the settings of each product you want to select the "default tax category" in the "tax category" setting.
Then, everything else happens in the System>Taxes menu where you can see a listing of your tax rules.
There, you can use the "manage tax rates" button to access the listing of tax rates. Make sure that you have your tax rate(s) configured there.
Then, on the listing of your tax rules, make sure you have at least one tax rule for that "Default tax category" tax category, and your tax rate for the zone where you want to charge taxes (or no zones if you want to charge taxes regardless of the location of the user).