how do you get a medal for a club?

Fail.

Submit some good content (Emphasis on the good content. Nothing duplicated, broken, spam, etc.) and hope that it gets 'fanned'[by other people who like your content] at a decent rate and stay active at the club. Sooner or later you will get your medal.

Contribute to it a lot. Add comments, answers, wallposts, good pictures, videos, etc.

link

This article pretty much sums up about medals.
you have to stay active, submit things in maybeevery section (videos, picks, etc.)