I got several sherlock holmes steam keys left and i thought it would be nice to have some sort of mystery giveaway.
Maybe even a murder mystery but i am not sure on how to do it plus if it involves a good story i am not good at it.
So anyone have some ideas or can help?

Ask Ghorpm, he can write something for you. :p

How about alibi challenge? People who want the game need to tell a story about what are they doing the night the murder happened. The most interesting and convincing story teller win the game.

Also you need to limit the story to 2 or 3 paragraphs.

Thought about it, but that would involve knowing the answers to questions people have, provide hints might get a bit of a hassle, keeping it in mind though.

Anyone else?

You could write out a short description with a list of persons of interest and ask people to enter with a guess as to which one is the culprit and then as the thread continues instead of bumps offer additional clues, until you reveal the answer and the winner.

Might be a bit difficult though if not written out in advance and as you mentioned if you don't write it yourself you may not have the answers if people are allowed to ask questions.