If you bought a vehicle in Germany, you will receive a letter from the Hauptzollamt. This letter asks you to pay for the vehicle tax (Kraftfahrzeugsteuer).

When you registered the vehicle, you signed a direct debit authorisation (SEPA-Lastschriftmandat). The Hauptzollamt will take the money from your bank account in around 30 days. As the letter says, you don't need to do anything. Just verify that they have the correct IBAN. If your bank details changed, contact the Hauptzollamt and tell them.

Look for a transaction from "BUNDESKASSE IN KIEL". The description should be something like "Kfz-Steuer fuer [vehicle plate number] fuer die Zeit vom 27.03.2018 bis zum 26.03.2019 Kassenzeichen [vehicle serial number]"