Fue is perfective, meaning that the action it describes are already completed (ex: Yo fue maestra hace mucho tiempo = I was a teacher a long time ago)
Era is imperfective, meaning the verb describes the duration rather than the completion of the action ( ex: Yo era un niño bueno cuando era más joven = I was a good child when I was younger.)
As a native English speaker, it took me quite a while to grasp this concept. The duolingo forum explained it really well. Here's the link: https://forum.duolingo.com/comment/200515/fue-vs-estuvo
Here are some more tips that I got from the forum to help you with fue/era:
Fue = Historical facts or things of the past.
» Abraham Lincoln fue presidente de los Estados Unidos.
» Yo fue jugador de beisbol hace cuatro años.
Era: for telling stories about someone.
» Abraham era un hombre muy bueno.
» Pablo era un hombre muy adorable cuando era más joven.
I had the same question , why no article 'a' before camarero? I have read the comments and really appreciated Marcy65brown's explanation. But now the question is why didn't DL explain this? This easily could have been in tips. If I didn't happen to read this post I still wouldn't know. This is a very haphazard why of teaching a language.