(If taxes are already included in your prices, make sure to enable that checkbox on the settings page above)
Once you have the tax rates setup for Germany, then any new orders will automatically calculate and charge the correct taxes for you, which will then be shown in your emails and order printer documents. This also ensures that customers see the correct tax when going through the checkout.
Hope it helps,
Bjorn Forsberg | FORSBERG+two | Award-winning Shopify Apps since 2011