

General rule of thumb that aligns well with what you do in English: “Sie” goes with last names, “du” goes with first names.
There are very rare exceptions, for example sports reporters tend to address some athletes with “Sie” and first name for reasons that nobody can explain. Those are not very relevant in everyday conversation, especially not if German is not your first language.
Is it a big deal to start using the informal?
It used to be a cliché that you would call coworkers by their last name and “Sie” until that one fateful office Christmas party where your boss gets drunk and asks you to call him Fritz and “du”.
These days, things are a lot more relaxed. Many companies are adopting a rule that all employees should address each other as “du”, including upper management.


My webserver had some spare capacity so I started my own instance.