Trigger for tracking and changing data. How to do?

Question

Database

Good day to all. Leaving for sick leave, the teacher gave the task - to write a trigger for the database in Microsoft SQL-server.
Own, the essence of the trigger - during the operation of withdrawing money, the funds in the client's account should decrease. If there is not enough money, a corresponding message appears. I tried to figure it out for a long time, but, unfortunately, it did not work :(