How to troubleshoot a malfunctioning starter in my Audi A4?

I have been having issues with my Audi A4's starter not engaging properly. Sometimes it works fine, but other times it makes a grinding or clicking noise and the engine won't start. What could be causing this and how can I fix it?

Have you checked the battery and electrical connections? The starter relies on a strong battery to function properly. Make sure all connections are clean and tight.