Welcome to Vienna!
You can find general information about Adventurer League here.
Basically it is standardised (you can play with your AL character anywhere in the world), organised D&D 5E play.
We meet every Friday and it is a really lovely, diverse community. Both experienced and new players are always welcome. We only ask that you announce yourself if you plan on coming so we can make sure we have enough DMs.
See this thread to get an idea.