how to display a list of OG content depending on user group membership? I've set up an OG Group type, and and OG Content type where I want to store some kind of articles.
I've provided fields where the author of an article can specify if an article can be:
1.  read by anyone, even unauthenticated users.
2.  read by all authenticated users, wheter or not they belong to a group to which the article is linked
3.  read only by users that belong to a group to which the article is linked.

I understand this has something to do with filters and relationships, but I can't get it to work....

Please help.... :-)
There is a pretty good tutorial of OG at https://drupal.org/node/1603460. 

If you already know the basics and have provided proper access fields using the Organic groups access control module, you should next explore the views that come with OG and edit them to your liking.

Also the OG Extras module at https://drupal.org/project/og_extras has lots of stuff for practical use.