Hi,
By default all your products should be created with the "default tax category" set in their "product tax category" setting.
So to configure taxes to all the products all the time, you need to do two things:
- go in the System>Taxes menu, click on the "manage tax rates" button, click on the "new" button and enter your tax name (VAT for example) and tax rate (20 for example for a 20% tax).
- go in the System>Taxes menu, click on the "new" button, select the "default tax category" in the "tax category" setting, select your tax rate you created in the first step, turn on the published setting and click on the "save" button.
That's it, you now have your tax rule add the tax rate you created on all your products.
If you already have a tax rate applied on your products and you want to change it, go in the menu System>Taxes, click on the "manage tax rates" button, edit your tax rate, change the percentage of the tax rate and save.