How to diagnose and fix a faulty starter on an Audi A3?

My Audi A3 won't start and I suspect it might be an issue with the starter. What steps should I take to diagnose and fix the problem?

First, check the battery and make sure it has enough charge. If the battery is fine, then you can move on to testing the starter itself.

To test the starter, try tapping it lightly with a hammer while someone else tries to start the car. If it starts, then you have a faulty starter that needs to be replaced.