Allow me some unusual contrarianism here, Andreas.
While the below is sound advice
...the discarding of those not opened after cooking is based on a myth. Better safe than sorry, obviously, but see for instance
http://www.abc.net.au/science/articles/2008/10/29/2404364.htm
As for the cooking and what to add, from my purist, frenchy point of view, it’s butter, butter, butter, chopped garlic and shallots, a splash of dry white wine, mussels in along with plenty of fresh thyme, shake around a little, steam until all (or most

) are opened. Full cream at the end is optional. I’ve tried plenty of other ingredients but always return to the classic.
Mussels is a typical summer meal in Norway, but as the r-months rule dictates, this is actually the sub-optimal season for having them. Mid winter they’re much better.
(
As uric acid runs a little higher in the family than ideal, a weekly mussels meal washed down with a few Belgian beers would not do me very good...)