Mitt jävla svar:
Scheiß- as a prefix is *very* common, it is a bit like fucking, bloody or (in Swedish) jävla. Fahr deine Scheißkarre da weg. Move your bloody car (Karre is pejorative for a car) out of the way.
Mist (=manure) is more okay to say than Scheiße. If you drop something and it breaks, it would be acceptable if a "Mist" escapes your mouth even in front of an audience. Though it technically denominates the same thing, it sounds less harsh, so it's a bit like the difference between to fuck and to shag.
Things to call someone whose behaviour is decidedly not okay: Arschloch or just Arsch, Wichser (like wanker).
Things to call someone who is dumb: (harsh:) Idiot or Trottel, (less harsh:) Depp, Honk or (ironic) Held.
Recommended, original swearing:
A friend of mine likes to replace verdammt by verdampft (evaporated), which sounds rather cute.
Elektroneger denominates a white person with a lot of artificial tan, especially if it doesn't look good on him/her (as is often the case with blondes). Is this word racist? I hope not, because I rather like it...
How common is swearing? According to Swede who moved here a long time ago (i.e. my former Swedish teacher), Swedes swear a good deal more than we do. From my own experience I can only say this matches my observations from radio and TV.